yes those are the peek nuts you want. They don't sell stainless steel nuts and screws at solidspot. You'll have to look for another source. Obviously since you are getting m2 peek nut insulators you also need m2 nuts and center post screw.
Stainless steel should be a healthier option but it's not as conductive as brass. If you decide to go that way make sure you get a flat top screw to have as large contact area with your battery as possible. If you need standards DIN84 and DIN963 are good options. To get too technical, the heads of some DIN84 have a sharp edge while others are a bit bevelled. That shouldn't matter but I don't have any means to make such measurements
. Just in case prefer the ones with the sharp corner, if you have that option. DIN963 heads are countersunk and will have as much area as the sharp corner DIN84. But because they are countersunk you *may* have to add another o-ring (I doubt it, don't get overworked with it).
I ordered mine from ebay. There were available locally in some hardware shops but they didn't want to bother with my limited quantity needs and were rude .......s. You may have better luck
The little screws are also m2 and they are 3mm in length. I replaced those with SS as well. Conductivity doesn't matter here as you effectively press the wire against the body of the RDA. You could even go with titanium for those but that's overkill
