1995 k1500 350 tbi to 5.3

I'm looking to do an LS swap into the above mentioned truck to replace a tired 350tbi with a blown head gasket. It will be my daily driver.

The 5.3l LM7 looks like the best engine for a strong upgrade to this truck from what I've read.

1-Where is the best place to purchase a good used engine and ecm?
2-How many miles is too many in your opinion?
3-what's a good price for one of these engines ?
4-what can I expect to spend overall if I want to do this swap as stock to stock?