I would go with the stainless tubing and some safety silv flux, and some safetysilv solder. I use the 45% stuff, and I think it is OK for what you are doing. I didn't have any problems doing stainless parts. Tigging stainless the first few outing was tough, but I didn't have any problem on my racks, or other fittings. You will find lots of it on ebay.

SS solves a lot of finishing problems. The powder coating can cost as much as the racks would cost to buy. It's worth it compared to custom racks, and to get what you want. But if you go stainless it costs about the same as 4130, and can be finished bright. It is not as strong as 4130, but some pros use it for racks, and the racks have normal dimensions and strutting. Another option is 4130 with some home paint, and stainless wear points. One home paint is appliance epoxy with the finish heated to about 160 to harden it off. White had an interesting rack that used 5/16 tubing, and 3/8 tubing where the bags go on, a short sleeve of it. That allows the 4130 to be painted with low wear enamels, as frames are painted. You can also do the eyelets an mud guard points in stainless.

Going straight propane is OK ish, better to go to two torches, than to go super fancy. For your rack the one torch should be fine. MAPP is a mixed blessing, it is slightly hotter, but it seems to contaminate more. Both MAPP and propane worked fine. At first when using MAPP I thought it was the solution, but later I returned to propane.

Propane is an entirely normal thing for silver brazing, but the parts you can actually handle are small. I would not waste money on a cheap OP system or OA, once you want to go there, get a good one, they aren't that expensive, and the performance is night and day. One really doesn't learn all that much about brazing bike parts with propane it is so bad. You might learn you like it enough you are willing to go all in.