Mosins aren't cheap junk, if you can get a good one. From what I have seen, and heard, just grabbing one at random probably won't get you a good one, though. But considering that there were about 14 million made (IIRC), there are bound to be a lot of bad, and good, ones.
As for the M48, yes, that too is another great gun. Only gun I ever regret selling.
K31 -> .308, I have heard of it being done, but IIRC only a few gunsmiths did it, and the money it cost you could buy a lot of 7.5x55.