If you're buying a used tank, ask when it was last Hydro'd (Hyrdo static testing). The longer it's been the sooner you will have to have it tested. Ask if it's 3yr, 5yr or other (Time between Hydro tests). Get a good visual inspection of the tank, make note of any damage to the tank. Scratches to the paint aren't bad, but if it's a deep gouge, it could be a problem down the road.
Burst disks as stated are a great safety feature and will scare the crap out of just about anyone the first time one fails. But I have seen valves go and when the valve goes, you don't want to be between the tank and where it wants to go.
C02 is more prone to changes in tempurature than Compressed Air. But when you compress C02 or release it, it going to make the tank cold. If you get a compressed air tank, becareful to slowly fill the tank, flash fill (filling very quickly) can be very dangerous. Also NEVER oil the input side of a Compressed Air tank. Once you have oil in a compressed air tank you have a bomb. Filling the tank will atomize the oil and heat from the compressing of the gas will ignite it.
If the package that you're picking up is in good condition and the price is right, jump on it. Just try and keep the tank at a constant tempurature and point the Burst Disk relief hole away from your passengers. Having a couple of pounds of C02 dumped into you won't make for a fun ride.