The cheapest and easiest platform to work with is a TM VSR (JG Bar compatible as well). You'll want to start with hop-up and barrel upgrades for accuracy and range. Then move to power/reliability by working on your piston, spring, spring guide, piston head, and trigger sears. Some people will suggest a new cylinder, but in actuality cylinders aren't a necessary upgrade part until the one you have just doesn't cut it anymore (or breaks). I'd be happy to talk specifics with you if you want to PM me.