hey welcome to the sport
MASKS
having owned or worn most of the masks around nowadays i'd say that the most important things to me are the comfort and the lense system. if a mask isn't comfortable to wear then no matter what else is good about it it's not worth wearing. after playing in some of the most horrible conditions imagineable (thank you scotland), the quality of the lense in how well it retards condensation is a big plus to me. on top of this if a lense is easy to change it makes life much easier.
with that in mind my personal favourite masks are dye i4's, vforce grillz.
i4's: best lenses for not steaming up, easy to change. very low profile and comfortable
grillz: not as good as i4, but still great lenses for stopping steaming and also easy to change. slightly larger profile but softer than i4 and very badass looking!
sly profit: super comfortable and easy to change lenses. dual pane lenses the same as i4 so don't steam up easily, but harder to get a hold of lenses in the UK (not a huge concern)
JT proflex: most comfortable mask but lenses aren't great. ear protection is pretty rubbish but very customiseable mask for a unique look
LOADERS
there is no point in buying a low end loader because i can guarantee as soon as you get into the game you will need to upgrade. best things to look for are speed/consistency, ease of use and maintenance.
dye rotor: low ball capacity but completely toolless disassembly (so so simple to clean and rebuild), low profile and has an antijam system
empire prophecy: my personal favourite. same low profile as rotor but much larger capacity. sound activated, incredibly easy to change from lid to speedfeed. toolless disassembly but a little more complicated than the rotor. has antijam freeway and ripdrive for unjamming.
invert halo too: amazing loader for the price, sound activated, has antijam freeway system (i've never jammed paint in this). requires screwdriver to disassemble and change lids.
TANKS
generally it is a standard recommendation to get a carbon fibre 68ci 4500psi tank since buying anything less (like a 3000psi steel tank) is less comfortable, hold less air, is heavier. The regulator isnt a great concern as all regs nowadays are at a standard that is suitable no matter what level of paintball you play.
PMI are a slightly cheaper tank but are readily available at a cheap price but are a little heavier than the high end tanks. you will also find they are a little longer than other tanks
Guerilla: the lightest, retestable tanks available. they also offer the myth g2 reg which allows you to position the guage and fill nipple wherever you like which is great for getting a low profile setup.
Stako: best tanks around. they are the most expensive and have the shortest life of just 5 years with no retest. however they are so incredibly light and are a little shorter but fatter, and for me personally this makes them very comfortable. there are new 68 and 48 tanks being released soon so keep an eye out! you will need to buy a regulator seperately.
ninja: DO NOT BUY THIS TANK. although they are reknown for being great tanks they are not PI tested and since this is the requirement for using in the UK you cannot use it (legally) in the UK.
MARKERS
i always recommend etek for a first marker because they shoot great, are reliable as hell and keep up with any other high end gun.
other guns to consider probably second hand are: empire axe and Proto PMR
these guns are in a simillar price range and although of a slightly different design offer alot of great features. most guns nowadays offer the same good features so do your research on what gun you think you would like the most. the best thing to do if possible is to get out and shoot them and make a decision.
things to really make sure you look for are: eyes, on/off asa, clamping feedneck, ease of maintenance, adjustability. if guns aren't meeting these requirements then they probably aren't worth getting.
personally the best companies to go with are: planet eclipse, dye/proto, empire, vanguard. depending on your budget i'm sure you will find your ideal gun, but you won't go wrong with an etek3
hope all this helps, im sitting in the house bored and had nothing better to do so this should cover everything you need to know!