Ive been looking around trying to figure out which optics would be "proper" on a MP5A5? I would prefer to co-witness with the iron sights, but it appears that something like this would be the only thing that would?
They make a raised mount for the mp5 that allows use of the irons 'thru' the mount, while allowing the standard rail mount to put any optic you choose on it. Personally, I like the 553 on an mp5, as it looks less bulky on a small bodied replica.
Yeah nothing will really co=witness with an mp5 in my experience, however optics on an mp5 can look pretty hot. I'd recommend either a smaller model eotech along the lines of the EXPS-2 or an aimpoint micro T-1. The T-1 is small enough that it wouldn't look bulky on the small mp5.
When looking at what type of rail/riser you wanna get, make sure you get one where the RIS rail is above the attaching system. The stock TM riser won't let you use many optics because the bits that mount to the MP5 upper come to close to the RIS rail, blocking spots on the rail. Hope that makes sense, but make sure that the rail's mounting system won't interfere with the optic mounting on the rail.
As far as optics go, the Eotech 551/552 are/were pretty common in Europe and in my opinion look pretty slick as well.