actually a syphon is for pulling liquid from the bottle, this is only required if you want to fill other tanks like paintball tanks. The question of owner bottle has to do with legality much like keg ownership, if the bottle was a rental then you will have to pay rental to the rightful owner and a gas supply shop will not fill it without you doing so. They may not even fill it. If you buy it from him and it is owned by somebody else then he had no right to sell it to you. On the other end of the story gas companies dont really care what is in the bottle as in helium, oxygen, argon ect. if you bring them an owner bottle and it has helium and you want co2 they will just trade you bottles and you pay for the gas, almost all the bottles of the same size cost the same to make and buy it is the contents that vary in cost. I hope this helps clarify, and I hope you can score this bottle if its a good deal. Good luck
P.S. also the bottle threads are differant between co2 and helium your reg wont fit