Hey Bob: a lot of guys use the alloy frame to build pistols, and they last. My lightweight Commander in 45 has at least 10k rounds through it, mostly ball and reloads, some pretty hot.
I would check the serial numbers for date of manufacture. Mine is in the first 2500 production run and although loose enough to feed most anything I can put into it, it is still accurate. I recall some frame cracking concerns or findings but am not solid on the specifics
"Can the frame material handle +P Super loads without cracking?" Yes, but the 38 Super does not need +P loads to perform. Most any handload from either current or older manuals is still within pressure specs for the cartridge so you should be well served with whatever you buy or load. Many factory loadings are loaded down just in case someone fires them through a older piece or one chambered in the 38 Auto (same case dimentions, just lower powered.) Actually, the first 38 Autos were loaded to what the 38 Super is loaded today, about 1280'/sec. The first 38 Supers were much hotter, running near 1400'/sec but then the concern about mixing loads and guns came to play and the 38 Auto was loaded down and the 38 Super was loaded down to original 38 Auto specs. A lot of the competitors using 38 Supers load to 'major' power levels and use fully ramped and supported bbls for their loads.
"If one of these is in good structural condition, can a stiffer recoil spring and rubber buffer add life to the frame?" You can always go with the stiffer recoil spring, say 18-20 lbs, but I would avoid the buffers as they tend to shorten the extraction and ejection stroke and can cause you problems.
There are a few places that will refinish the alloy frame but often they require a bunch of guns to be refinished so they don't have to cook up a batch of the alloy finishing material for only one gun. Checkmate in Florida used to refinish alloy guns but I don't know if they are still in business. HTH.
