[L473ncy]

+1 to goggles first. ATGATT, parts can be replaced/upgraded eyes can't. Depending on what your local venue allows (PB goggles+mask, PB goggles only, Sealed ballistics, ballistic glasses) you'd be looking for something that fits that. PB goggles + mask are always good for anywhere but you may not get as good of a cheek weld and not be able to aim down the sights as easily. I highly recommend the JT Proflex or Flex-8's and I've seen a lot of people cut away at the material so there's plastic protecting your teeth/mouth and bare cheeks to be able to get a decent cheekweld on the gun. Otherwise Revision and ESS are what a lot of people seem to use.

Whatever other safety gear you need buy it (good boots are recommended but if you have good hiking shoes you can use those for now, and I mean GOOD hiking shoes with ankle support and preferably a steel shank to add extra support if you have weak arches). tl;dr Treat your feet well cause you run in them and abuse them all day, especially if you're active and moving a lot.

Now gun; CYMA AK's are supposedly BEAST and with a teardown, some precautionary maintenance (and maybe a spring downgrade to 400 FPS or 350 depending on your local venue's rules) they can easily last 100K rounds stock. Doesn't mean you won't get a lemon because the QC isn't that great coming from a mass produced factory in China but they're pretty good overall. For M4's (AR15 platform) you'll want to go with Jing Gong or King Arms.

With a $600 budget you're totally in luck and can get mostly kitted out for your first season or two before you get better stuff. Let me give you a quick breakdown of some costs;

Goggles: $60-70
Boots: (if you don't have) $100
BDU's: $50 for a set of CF Olive Drab from a surplus store
Gun: $200-250
Battery $15-20 (from hobbyking or Airsoftstore.ca)
Charger: ~$30 (from hobbyking, let me know if you need help picking something you can go simple like iMax B6/B6AC/Turnigy/something more simple)
BB's: ~$60 for like 4 bags of 5000 ct. (20,000 should last you like a season or more depending on your play style) (BBBastard, Excel, Madbull, KSC Perfect, Airsoft Elite, Evike/Matrix are alright as well (I'm 99% sure they're OEM by Airsoft Elite))
Field fee: ~$20 (unless you play at a private venue for free or whatever)

Total comes to $600 ($580 if you don't include field fee), I feel I've missed something but otherwise that should get you good to go and if anything else is needed then your budget might need to go to $650 or so but that's the basics I can think of.

If you need upgrade parts definitely go to Stealth (he's a user on this forum and the owner of Airsoftstore.ca) and get Lonex or SHS or Core parts, they're cheap, high quality and work. Also note that price isn't typically a reflection of quality, Lonex is high quality and cheaper than most other brands. Failing that Modify and Guarder make pretty good stuff too (I have a lot of Guarder parts in my guns because that's what was easily available to me at the time). If you need a spring downgrade to get to field limits a spring is $10-15 and labour for someone to do it for you shouldn't be more than like $20-25. Also remember limits are LIMITS not goals, a 380 FPS gun is just as good as something that shoots 399.99 FPS time to target is more or less the same and depending on other upgrades (mainly for consistency) range and accuracy is also more or less the same.