Teflon said:
4500psi more shots a 1.1l 4500psi is your best bet second hand you cna pick em up for about £75 brand new £110-£140
You'll be luck for that price, normaly 90-100 for a 2nd hand PMI HP 1.1 4500psi tank with approx 3 years test left on it. Bit more for a crossfire.
0.8 3000psi aluminium tanks 2nd hand £30-40 and about £55-60 new.
1.1 300psi fibre wrap will set you back £100 new or about £60 2nd hand.
as the other guy said you will get more shoots out of a 4500psi tank but only if oyu can get a fill from somewhere like H-pac a dive tank will only give you about 3000psi.