^ There are really just two ways to maintain the guns and both methods are costly and REALLY suck.
1) Use high quality parts and pay a knowledgeable and established gunsmith to fix them (of which nobody wants to do 12 guns a week lol), so essentially drop $200 (lonex is perfect balance of inexpensive and high enough quality) into each gun plus gunsmith fees. Downside is, every mechbox eventually breaks anyway, noobs can break guns and I mean in half wrecking all the parts, and all noobs continue to shoot when the guns jammed so guranteed stripped pistons.
2) use only stock parts but buy rental guns that are decent enough to be rented out. Downside is now they're stock and guaranteed to break down more often than the upgraded ones, but the parts are probably much cheaper since you probably have a dealer account.
Anyway, total agree 100% with everything above, you can't invest too much in something you know will be broken constantly and mishandled by the most inexperienced people. And you can't spend too little like buying G&G or you'll be going through metal bodies like crazy.