Stainless gets an oxide film that interferes with braze strength. To some extent the chemical action of the flux can dissolve the oxides, but you want to minimize the amount of work for your flux to do. So starting with freshly cleaned stainless will give better results.
I'm not an expert, just a long-time (~50 years) practitioner. I don't know how fast the oxides form on SS at room temp. But I think letting them sit overnight might be getting into sub-optimal territory, so I would scuff up the SS right before brazing, with some abrasive cloth ("shop roll"), or scotchbrite. Abrasives can leave residue, so an acetone wipe after the abrasive scuffing is probably worth it. My fastidious prep for SS for brazing might be overkill, but I do get gorgeous braze joints on SS with perfect penetration and little to no burnt flux.
As to gases and tips: I used acetylene for 10+ years before switching to propane. I still have an acetylene bottle in the shop but it seldom gets used. Other than gas-welding, nothing I do needs acetylene, and I like propane just fine. For O2, I use an oxygen concentrator, the medical kind. Got mine used off Craig's for $150. With a concentrator, I don't have to refill O2 tanks ever again, and with propane in 20 lb bottles (gas-grill style), I never have to go to the welding-gas supply store anymore. Obviously 20 lb bottles wouldn't last long in a factory, but I am a retired hobbyist. Keeping several propane bottles around is cheap enough, and swapping in a new one when one runs dry is pretty quick..
I use the same torches for both O-A and O-P, but propane-specific tips keep the flame from detaching and blowing out, a common problem when using acetylene tips for propane. The tips I like best are made by
Paige Tools in Seattle. They have a ring of small flames surrounding the main center flame, which makes the flame very stable and concentrated. It's not like a rosebud (tho Paige does make true rosebuds as well); the tips I'm talking about make a flame shape similar to a single-orifice O-A flame. Good pinpoint heat control.
A quick'n'dirty method for getting a propane flame to stay sttached with an acetylene tip is to counterbore the orifice. Hard to do precisely by hand, better if you have a lathe. Somewhere on the web they'll tell you the diameter and depth to counterbore them to, but I just did them by eye, bore about 4x the orifice size, and about that deep as well. Doesn't seem to be too sensitive to bore diam and depth. I call it counterboring, but I just used a regular twist drill, doesn't need a flat bottom. I think you can buy tips that have this done to them already, but if you're going to buy tips, just get the Paige tips, they're godlike.
