For making small joints as in racks stainless & silver is not that finicky but you will need practice for sure. Go for 0.035 wall; stainless (especially 304) is generally weaker than 4130 steel.

Nickel-Silver is a filler that melts at brass temperatures. To join with stainless you need a special flux. It can work with normal brass-brazing flux but it's much harder. Cycle design has some.

I haven't looked at McMaster Carr but Aircraft Spruce is a good source for steel; they have the basics for racks in stainless as well.