I don't know about your part of the world, but in mine, there aren't many options in tank size, just get the largest, not terribly large. Some regulatory thing. Search on torches, they have been heavily discussed. You can get by with an 035 and 050 tip. There is a chart somewhere of all the corresponding sizes. I like the meco torch, but not the best option for pro use.
Another option on gases is propane. Good for bikes, and less dangerous, and less dirty. Ideal would be a propane system and a medical oxygen concentrator, or at least it sounds like the ideal... Typical is obviously A/O.