Awesome guns out if the box, spend your time shooting the longer ranges and challenge yourself. There more gun there than you think. Practice as much as you can.
Anybody whos done enough shooting knows you can't compare apples to oranges, this forum is dominated by revolver shooters so you know where that will lead. You will find the average $600 dollar wheelgun is more accurate then the run of the mill $600 autoloader especially the plastic ones...
IMO you're over-thinking it, either one will serve you fine just make whatever mods you desire to a MK3 and have the benefit of a better mag release position, something you cant modify easily on the MK2.
Amazon is selling reprints of Sixguns for a reasonable price. I have at least a hundred in my collection and am still looking for the perfect one, Sixguns by Keith is one of the best, Taffins by the same name is good too.
I've had 180g fmj in my G20 at 1346 with a heavier spring than stock, 22lbs IIRC. I think with support the RH has you can get more power out of it. There is a guy I remember posted loading some really hot stuff in his 357RH, "5of7" is his name, get in touch with him.
Sounds ridiculous. I wouldn't shoot anymore out of that thing if it really is a 3 screw until you figure out what you got there. The old 357's were built on a smaller frame which mandates a smaller diameter cylinder that wouldn't be safe to shoot 44mags out of. Can you post a pic?
I'd keep it out of the receiver unless you want to clean it back out. The finish on a mark3 would have to be pretty bad before I'd play with it. Stainless is the ultimate finish, it won't wear through, my conservative opinion.