Blowby?
I got mine this spring, 92 4x4 auto 1ton single wheel pretty good body with no rust for 3500. It had some blowby, but I didn't worry about that too much. My local diesel wizard said it wasn't enough to worry about.
Unfortunately, it had been over toasted and cylinders were scored. After the headgasket blew and I took it apart, I am up for a rebuild. I didn't get that good of a deal. Since I can fix it I will be OK, but not a great deal.
From my limited experience, if mimimal blowby and no bad sounds from driveline in 4x4, probably not a bad deal.
IMHO Pros, you should be pre PCM computer, IE less to fail later and less complicated.
Cons, unless you know the truck and its owner, you never know.