The newer sysytems usually have backflow preventers on them, MAKE SURE! It is possible to have flame backflow into the acetylene tank, since the oxygen is a higher pressure.
I recently found an okay source for tanks on ebay from Indiana Oxygen, I paid $240 including shipping. The tanks are on the small side 80 Cu ft oxygen and 40 cu ft acetylene. I don't weld with this but it makes cutoff of larger stock much easier. Of course I also have a TIG, MIG, 240 v Stick welder and even a small 110 v stick welder. Each has their own special applications, though I use the 240v stick welder the most.
Used welders are out there, you just have to look for them. My last acquired was the MIg and I paid $25 for it because the guy had lost the nozzle and could not find a replacement. I took out my handy dial calipers and measured it out and then went on a hunting spree. Everyone said the only place that you could get a replacement for Chicago Electric was from some defunct company in California and that no other brand would work. I'm happy to say that it just ain't true! Chicago Electric is not the greatest, but they are not bad welders for the money(especially at $25 for a normal $350 model). :D