Potential donor truck

[FL71C10]

I am still searching for an eninge and transmission for my truck (71 c10 swb 2wd, was originally a v8 automatic truck). I have found a few that I am trying to decide on. Maybe you guys can give me your opinions:
1) I found two motor homes with low mileage 454's in them. One is a 1990 with a carb and a Turbo 400, the other one is out of an 86 motorhome. They both have less than 60k miles and I think both of them are still in the trucks and I can see and hear them run. One is $700 and the other is $900.
2) I have found a few vans with 350's that have around 90,000 miles for about $500 each.
3) I found a 1994 suburban frame with a 350 automatic 4WD. They guy took the body off and may not have all of the wiring or other parts. I think this is a problem waiting to happen due to the potential missing engine computer accessories. $500
4) I found someone who has a turbo 350 transmission that was rebuilt by Aamco in 2004 but he has no paperwork. Don't think it has been run since the rebuild. I thought I could find a small block to use with it but didn't know how longs the seals etc would last in a rebuilt trans. Whether or not it was actually rebuilt or not, who knows?

I'm not a big horesepower guy and I think I would be fine with a small block. How tough is it to put a big block in these trucks and which option (or neither one of them) do you guys think would be a better option?

How much is a crate motor?

Thanks!