On the gun you'll have to decide a few things one the type of platform whether gas or electric. I'd argue that electric is more practical, but less realistic. Gas is more fun, but a heck of allot more work to keep running and bar some examples doesn't do as well in colder weather. For myself that puts me on the electric side of the equation since I play year round.
Either way will give you semi/full generally by default because that's the most common fire modes. But electric I know with certainty you can swap to burst using a simple mosfet, even inline. So don't base your initial purchase on that part. Base it on the quality of the platform etc. On GBB I'm not sure but probably most would have an option for burst, I do know the polarstars and that HPA side do, but I'm not sure on the typical gas guns I don't have much experience with their workings to say.
@T6e9a would probably have a better idea there.
The Mags, depends on platform but AEG mags are much cheaper. So add that into the cost idea for the whole setup and how many mags you want. Coming from real firearms(my background/current as well), generally speaking in most events you'll use far more ammo than you would with a real firearm. One BB doesn't equal one round of ammo it's just not that effective except in certain scenarios. So expect to double or triple your ammo consumption you'd expect from a real firearm. Again my 2 cents, but that seems to be the case most of the time.
Batteries, see the other post;)
Eye/face stuff, I like mesh masks just get a steel one that fits fair, then bend it to the shape of your face so it doesn't create hot spots and they work pretty well stopping teeth damage and the like. Eye's you're going to be looking at some sort of rated full seal. I'll let other guys jump in there, I own allot but most of my eyepro is old and no longer available. But ESS and other goggles that were used overseas are a good option if available. But fogging may be an issue, at a minimum get a thermal lens goggle. Which has two layers like a thermal window in a house. That helps with fog but may not solve it. I've got a post here were I built a fan system into my helmet to solve my fogging issues.
I might have more but that gives you a start;)
Luke