I don't claim to be a total expert on gas guns but I've figured out a few things.
1) TM Hi-Capa series. Solid pistols and upgrade parts are plenty available. If you have any problems Illusion is the resident expert on them. Personally I like the 1911 MEU (AFAIK some/a lot of parts are swappable between the 1911/MEU/Hicapa) since they're all "1911 based" (the Hi-Capa just being a "2011" double stack "race" gun for competition shooting).
2) I would say hop up and probably hi flow valves. 9Ball supposedly makes a kickass hop up rubber but as of late I've heard comments about how their quality has gone downhill. Other than that see if you can buy parts off Illusion, I'm pretty sure all the stuff he carries is the highest of quality.
3) Understand the system. Basically it's liquid -> gas expansion that propels a BB down a metal tube. In between there's various springs that release other springs and release the hammer down to hit the dump valve on the mag which in turn dumps a set amount of gas into the chamber area pushing gas backwards (giving blowback function using Newtons laws that every action has equal + opposite reaction, in the case of NBB's since it's "rigid" the gas can only escape in one direction and all of it goes towards propelling the BB) and forwards (to propel the BB).
4) I honestly just take it and play with my pistols as they were out of the box (or in the case of used pistols how they were sent to me). That doesn't mean that maintenance isn't important, I relube and clean my slide, slide rails, mags, etc. once in a while. Just watch this video if you want to do regular maintenance:
Gas Blowback Regular Maintenance - RedWolf Airsoft - RWTV - YouTube
Note that I personally use 100% 25wt. silicone RC Shock oil. A bit thicker than what most others use but it's "stickier" and will last longer. You can go down to 15wt. if you want but the AI stuff IIRC is something like 1.5wt. silicone oil. Also a bit of a weird thing that I found works is chain lube for bikes (ie. Finishline Teflon dry lube). That stuff is amazing, I highly recommend them for AEG gears if you're using your AEG in "dirty" conditions (based off biking definitions). Basically I've found that if you play in dusty conditions regular AEG greases tend to attract dirt and such. A solution to this is make sure the lubes you use are "dry" and that means things like Graphite powder and Teflon drylube (I'm thinking of trying out the Finishline Ceramic Wax lube next but that will be a long time before I use up all of my other chain lubes, even if I regularly lube up my bike chains).
5) Ef-Pee-Ess, Ef-Pee-Ess, Ef-Pee-Ess. You don't need to upgrade to hiflow valves, rocket valves, recoil springs, and all that to be shooting 400 FPS on your pistol. After all it's supposed to be a sidearm for engaging below MED's. The only time you should be aiming for 400 FPS or something like that is when you're doing a proof of concept project.
6) Boots, eyepro, BDU's, chestrig, pouches, good mags, gloves, sling, etc. Once you have all that stuff then delve into pistols if it's something you really want. To be honest I've only pulled out a pistol maybe less than 10 times in all of my airsoft career (not counting pistol only games) and gotten maybe 3 kills with it.
7) I highly recommend the cansoft TM Glocks if that's your thing. The price IMO is a bit high at the moment but I suspect that the prices will drop down to around ~270 for them once they become more commonplace. Yes it's not as cool as the black guns that you can get if you pay a little bit more for but if you're looking for something solid that OOTB will work and you don't plan on replacing everything it's a pretty good deal. If that's not your thing then feel free to peruse the retailers section. I recommend TM, KSC/KWA, Socom Gear, and Western Arms Magna/SCW/2/3 (but this is more of a collectors type gun than anything).
Also avoid WE, on occasion you'll get an awesome pistol but the QC IMO is just not there, is hit and miss, and can widely vary. Someone might get an awesome pistol from them but if you buy another one from a different batch it might suck. I've seen my fair share of WE's with problems but I've also seen some pretty good performing WE pistols too (although I didn't directly compare them it was probably on par with TM stuff).