General rule: Buy from trusted brands. From what I've gathered, a lot of these guns will need to be tuned/fixed/upgraded/downgraded/broken/fixed/brokenagain/fixedagain before you can even use them. For your budget I could recommend G&P and G&G as they are overall good companies. And since all the ones you linked are M4/SR15 variants, try looking through G&P from a lot of the sites as G&P make primarily M4s. Otherwise I would recommend Tokyo Marui (As would 99.999% of the guys in the club) but they will most definitely be out of your budget.
I myself have a G&P Sentry and the only problems I had with it was caused by me from changing/cutting springs and generally not knowing how to take apart and put together a gearbox properly.
With alot of the DYTAC and random brands I think you will find you need new springs/cut springs to lwoer the FPS a bit but I'm sure someone will help you out with that. (-cough- sort of a self taught expert on G&P Gearboxes by now

)
Try looking on other sites like:
wgcshop.com
echigoya-guns.com (just to drool over guns that cost an arm and a leg)
EDIT: Also it may be a good idea to go all out on AEG as the only other thing you really need to get started on airsoft is safety goggles/glasses. Building your gear around a good primary is a good start but again, you can do whatever tickles your fancy
