I would like to base it off of a Slash 4x4 or Stampede 4x4, but are there any stainless steel bearing sets available for these?
FYI depending on the grade of stainless steel it can, and will rust still.
ceramic might be your best bet if seeing you really are set on using one of those models.
BTW there is one reason I wouldn't use either for your idea. The center drive shaft is in an inclosed area that mud can get in, and cause damage seeing the thing is covered by a clear piece of plexiglass that doesn't even form a seal, and is prone to breaking(this things plastic is WEAK). The one I bought I dunno what the dude did but something got in it and damaged the center drive shaft. Where as a crawler, WK, and others with open drive shafts won't have the issue. I'd even use a tamiya DF-02 with MT tires on it over the slash as the center drive shaft is enclosed better so less can get in