It's been a while since I've been in the paintball scene, so correct me if I'm wrong. Anyway, this is what I remember.
HPA and "nitrogen" are the same thing (air is 78% nitrogen anyway).
Firstly, what type of marker are you hooking this unit up to? Some markers are high pressure (HP) [some autocockers, stacked tube open bolt blowbacks (aka spyders, fishy's, ect], some are low pressure (LP) [timmy's, angels, some autocockers, impulses, shockers, ect]... if you hook a HP tank straight onto an LP marker, bad things will happen; a LP tank on a HP marker, and it simply won't cycle properly.
HPA tank kits generally come in 3 types: HP, LP, and adjustable reg. Unlike most (if not all) CO2 tanks, HPA tanks generally have a regulator that is the neck of the bottle (Exept in a few cases). Adjustable regs are expensive but have the advantage that they'll carry over to your next marker much easier (if you have an HP marker and buy a HP tank, and then later switch to a LP marker, you'll have to run a secondary regulator to bring the pressure down or change the tank regulator). If you buy an LP tank and switch to an HP marker, you're screwed.
HPA tanks, unlike CO2, require a regulator in some form or another. I have a Nitro Duck 4500psi 110ci tank (yes, it's a beast), the tank itself has a wierd 2.5 turn widethread nozzle on it that
only screws into the Nitro Duck regulator that it came with... but most average tanks' nozzles double as the regulator (as previously mentioned).
If you have a high performance marker that's hyper sensitive to pressure spikes, a secondary regulator in the foregrip is pretty damn good at keeping your selenoid intact, as that built-on tank regs aren't very smooth at delivering constant flow and constant pressure.
One last thing, is to always keep any oils far, far away from the fill nozzle of your HPA tank or bad bad things can happen. Also, avoid dropping your tank whenever possible and always have a good tank cover on it in the field... if you cut (fiberglass) or dent (steel) the tank, most places should (by law) refuse to fill it for you
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To answer your questions directly, as best I can:
What size tank should I get?
Depends on where you play and how long you play for. If you're taking +500 shots per game, a 68ci tank may not hold out if your marker isn't efficient. A 110ci tank is almost literally twice the physical size of a 68ci (and three times the length of a 68ci stubbie). If you're a speedballer that needs to play tight at the front, a smaller tank will benifit your game alot. The bigger the tank, the heavier the tank... but a 68ci steel tank weighs the same as a 110ci carbon fibre wrap tank. Carbon Fibre wrap isn't cheap though, but after a long day of packing around a fully loaded marker, your arm with thank you.
You can run remote lines if you don't want to pack your tank attached to your marker. Remember that a tank is an easy target and will break paint everytime... so having a giant tank can be a liability no matter where it is.
What PSI rating?
Last I knew, there was 3000psi/4500psi/5000psi. The difference is price. 5k tanks are wicked expensive, 3k tanks are cheap like bourche. Steel tanks are generally always 3000psi, carbon fibre is generally always 4500... When I last followed paintball, the only 5000psi tank on the market was the Thor 5000 system (*drool*).
Having a 5000psi tank is cool and all... but outside of tournament level play, there's very few places that can fill that high (A compressor for that kind of pressure is 30-50 thousand dollars, and loud as f&*k). Check around to find out what local places can fill tanks to... there's no point in buying a 4500 tank only to find out your local shops only fill to less than 3000psi. Scuba siphon fills are generally under 2500psi as well... so be wary of going full out on a sweet 4500 tank because there may be no use for that extra 1500psi.
So, in short, PSI ratings is a question of practicality and economics... if you can afford it, get a 4500 system if you can get it filled. Otherwise, don't bother wasting your money.
Where should I buy?
Someplace recommended by your pball buddies.
Cost is a concern, I dont wanna spend more than $80.
In all honesty, check around for some prices and figure out what you want. If you can get a used tank, go for it. Make sure it's been recently hydrotested and has no damage to its shell whatsoever.
If you have any questions, PM me and I'll do my best to help you out.
And Lastly, Brands, any prefrence?
All tanks are manufactured to DOT approved standards, so tank wise, it's tomato/tomato. Regulator wise, it's a whole different ballgame. Get what you can afford and upgrade later, I guess.
