Right...Dont buy an Aluminium Tank...Steel is better (Faber, who make Dive tanks, have the tag line "If it's Steel, it must be Faber!") You can get composite (Kevlar) tanks, but they are costly and not usually big enough.
Get on the phone to LPC (London Paintball) they will sell you a complete 300bar tank and rig for around £180. Importantly, the tanks are fitted with a DRY valve (ie: not designed to be submerged) and therefore, only need to be tested every 5 years.
Scuba tanks, require a visual inspection every 2.5 years, and a preassure test every 5th year.