If you are concerned about the risks associated with acetylene then you could use oxygen in conjunction with propane as a convenient source of heat for freeing siezed parts, brazing, cutting steel etc but not for welding as the flame is not hot enough. You would need a different torch from oxy acetylene though. Hobby weld gas is dearer than BOC etc volume for volume but as you know there is a rental charge. Hobbyweld will charge you a deposit ( refundable, but they may keep 10% or so) plus the cost of the gas. I recently paid about £115 for a medium sized bottle of argoshield, I suspect oxygen would be similar.
So overall the bottles may be of no use ( once empty) , and the torch may not be suitable ( although you may just need a new nozzle). Also I think you will need a different hose for propane, which leaves just the oxy hose and flashback arrestor, plus the torch with replaced nozzle - not a great deal really.
HOWEVER - you may be able to get the existing bottles refilled. BOC etc have tried to stop people from having unregistered ( and therefore un-rented) bottles but some suppliers may still refill (actually swap empty for full for the cost of thd gas), it depends on the supplier and 'helpful' suppliers are becoming increasingly scarce.
Finally, it is illegal to have these bottles unless the they are formally rented. They do not belong to anyone other then the company.
K