First and foremost, email airsoftclub to find out what they're putting in their guns. That's the only way that you're going to know exactly what's in it.
Now, if you're going to take the job on yourself, it's really not terribly complicated until you start tampering with the gas flow. So lets ignore that for now.
Let's start from the top, the gun that you want to start with. For competition shooting, you're going to want a double stacker. They hold more bbs and offer much greater gas consistency because of the larger gas reservoir. You're going to want to look at any WA 1911/2011 using a 25-30 round mag. Prokillers are full auto, and as I understand it, won't be allowed in any competition shoots because of it, so if IPSC is what you're going for, then forget those, too. Don't worry, you still have a lot of options left. The vast majority of upgrades for one semi auto double stacker 1911/2011 will work for all the rest, too. From all the rest to choose from, your first priority is to find something astheticly pleasing to you. But sometimes they all are, you know
! So give this little puppy a read through
home.netvigator.com/~colekwok/western_arms_hicap_comparison_chart.htmIt's a great comparison of the WA hicaps. Some of the newer ones might not be on there, though. When you're picking, you really have four main differences between all of the guns (and of course a bunch of smaller ones). The first is slide length. 5inch is pretty standard, but with a longer slide comes a longer barrel, and with a longer barrel comes more power and more accuracy. If you like the 6inchers (and remember, there are a lot less of them), then it can't hurt. You won't want to go shorter than 5inches with a competition pistol, you lose a lot in velocity and accuracy. If you don't have to worry about being tactical, you only have to worry about it being too front heavy to aim (for example
www6.wargameclub.com/pcart/shopper.php?itm=WA-PS-IFL_cat_WA%20Handguns 
). Keep in mind, there are tons more metal slides available for 5inch models than any other model.
The second factor you need to consider is whether you want the older look or a more modern look. The Para's, for example have more of a classic 1911 style to them where as say, the Xcelerator has a very new aged look. You can do the same things to either of them, so it's really based on what look you want.
The third thing to consider is whether you want a compensator or not. A compensator is a great way to add barrel length (and again, power and accuracy) and still use the 5inch slide, plus they give a pretty unique look. There is one problem that I ran into like a brick wall with a compensated pistol. Every Shooter's Design slide that I've handled has been a couple millimeters too long, and runs into the compensator prematurely, and you can't fire the gun. There is a good chance that you'll have to shorten the slide, especially if you buy a shooter's design (sd) slide. Sheriff and PGC slides might have a better fit, they are certainly more expensive. But no matter which way you go with your slide, prepare yourself for some filling. I'll go more into detail on slides later.
The fourth is whether you want a red dot sight or iron sights. Red dots are great things to have on pistols, but you may want iron sights for whatever reason. If you want a red dot, you'll want to get a pistol with a scope mount built in. If you change your mind after you buy it, you'll have to drill the frame and install one yourself, and I wouldn't want to take that job on in a million years.
After that, it's all looks. Full dust cover, half dust cover, sliver, black, two tone, it's all what you want.
Then there is one last thing to consider. Hop-up. It seems like a no-brainer. Hop-up is better, right? IPSC shoots take place in a very close environment, so you don't need the range. What you do need is accuracy, which hop-up only degrades. You may want hop-up, it will dramatically increase your range, and really doesn't impede accuracy too much, but it's something to consider. I myself chose a pistol with no hop-up.
www6.wargameclub.com/pcart/shopper.php?cat=WA%20HandgunsFor IPSC or really any sort of target pistol shooting, you're going to want to use green gas for the accelerated blow back and greater range. Because of that, you're going to need to upgrade some internals to take the extra force. First, you will NEED a new, reinforced loading nozzle (translated poorly on most HK retail sites as a loading muzzle). The stock WA part is very prone to breakage, it's usually the first part to wear out in the gun whether you're using green gas or not. PGC makes a great, near-drop in part. You may have to do some slight filling around the "wings." (on the underside, you can see them in this picture)
www6.wargameclub.com/pcart/shopper.php?itm=PGC-PT-DLM_srch_loading%20muzzleNext, the hammer has a tendency to wear out. It's not a necessity that you replace it right away, but it's something to look for in the future. I went with a guarder stainless steel hammer, and it's treating me well so far.
www6.wargameclub.com/pcart/shopper.php?itm=GD-PT-WA07SV_srch_hammerNow here is the somewhat tricky part. You're going to need a metal slide, and which type you can get will depend on your gun and how much you want to spend. There are a lot of options available, but your starting gun is going to cut them down. As I said before, if you want a 6inch slide, you're going to lose a lot of options. They just don't make that many of them. Now, if you choose a 5inch slide, you're going to be pummled with options. Again, three rules that must be met in order to find a proper slide for your gun. You must match the rear sights, if the pistol you choose uses bomar sites, you need a slide that takes bomar sights. It has to do with how the gas chamber is afixed to the slide. The second is whether the slide is ment for a gun with a full dust cover or partial dust cover. Choose the wrong style for the wrong gun and it'll just look wierd. This one seems like common sense, but choose a 5inch slide for a 5inch gun, and likewise for a 6inch slide to a 6inch gun. After you've covered all of those, you can choose any slide that meets the criteria. Shooter's Design (WGC calls the GRS, but it's the same company)slides tend to be a little lighter while PGC's are heavier. I honestly can't remember how sheriff slides come in in terms of weight. I want to say they're heavier, but i'm not sure. With a heavier slide, you may need stronger springs, but this isn't the place for that, i'll cover it later. PGC and Sheriff slides typically take more filling than SD/GRS slides in terms of a slide-rail fit (i really don't know how PGC and Sheriffs compare in terms of fitting with a compensator).
www6.wargameclub.com/pcart/shopper.php?cat=Metal%20Slide%20for%20HiCap%20and%20SV%20SeriesTake your pic, there are a lot of interesting jobs on the SD/GRS slides, so find something cool. The slide I chose was a Den Trinity Custom.
Now, I started to touch on it, springs. You might need higher tension springs, you might not. At their cost, i say buy them with the package, and sell them if you don't need them. This is what you're going to want to get.
www6.wargameclub.com/pcart/shopper.php?itm=GD-PT-WA02_srch_springFinally (I hope), you'll need to get a metal chamber and barrel set. I remember the least amount of information about these...sorry. Ask about them on
airsoftplayers and search around on
Arnie's Airsoft. Arnies has a lot of information in their guides. Give them a good read through.
I'm getting tired right now. I'll be back tomorrow to see if there is anything I left out. Unfortunately, most of the great information on ASZ, ASP, and AO were lost since all three have gone down. Arnie's airsoft and
UKAN are going to be your best resources for information regarding this stuff. Take your time and search around on those sites. If you have any more questions, ask away.
! So give this little puppy a read through
). Keep in mind, there are tons more metal slides available for 5inch models than any other model.