I personally prefer Ninja tanks over Guerilla tanks and you want the SLP version of whatever tank you get. SLP designates the tank regulator as low pressure and pre-regulates the output of the tank down to 300PSI from 3000PSI (in most cases). The tank regulator is not adjustable and is there because air lines can not handle 3000PSI.
The regulator that feeds the HPA engine is the line regulator and is adjustable (typically 80-120PSI for airsoft applications). The inexpensive option is the ARX20 and can be found for around $20 but is the least consistent. Palmers, Ninja, Redline, Wolverine and even Mancraft offer more consistent options that are usually around $60-$120. Some are rated for unregulated CO2, some are not. Be sure to get a regulator with a tournament lock so you can prevent field adjustment (most fields/events require one!).
As for tank size, that really depends on how much you plan on shooting, how difficult the tank will be to refill and how expensive refills are. My local field offers free refills with admission so a 13/3000 covers just about anything I run short of my HPA SAW for a longer scenario. If refills are not so convenient or if you are attending MILSIM games where HPA may not be readily available, you should consider larger or multiple tanks. A 48/3000 should be sufficient to see a DMR or sniper though all but the longest MILSIM scenarios, though a 62/3000 gets you a bunch more air while being the same diameter (3 5/8") and 2 inches longer. For the longest MILSIM games (like a 24hr BlackSheep game), a 4500PSI tank is an option provided you have the ability to get it filled to 4500PSI.