I agree with Lefty. Maybe you could talk price with him
IF it starts somewhere in the $450 - $550 range.
Of course, if you were all set to buy a M520 and have these exact modifications made, you can pay all you want for it with my blessing!
Personally, I don't buy the idea that it is worth what he has put into it. He may have upwards of two thousand in it. Anybody want to pay that much for it?
Not too long ago, there was a highly modified M520 in the Classifieds. The seller had Hamilton Bowen install a 5" barrel and convert it to .45 Colt. Of course it was refinished and looked beautiful. A 5" fixed-sight N-Frame in .45 Colt! Yes, it's a gun S&W should have offered as a standard factory model.
I dearly would have loved to buy it, but he was trying to get back all he had invested in it ($2,500 IIRC). Half that figure would have been a better starting point. He kept it in the Classifieds for a couple of weeks and then let it fade away with no takers.
Was it "worth it"? You would have to ask him. Didn't seem like anyone here thought so, at least not anyone who was willing to put his money where his mouth is.