used truck purchase advice

Sounds like it may be more trouble than its worth. For me the red flags are:

1. "engine was replaced with an older engine" So first off, the service manual MAY not jive with what you have and we have no idea what kind of condition this engine is in. Computers are picky. You take one engine with an 02 sensor and swap it with the same engine with an air/fuel ratio sensor instead of an O2. (They look similar.) You'll can have all sorts of emissions issues. I think it was Toyota who had O2 or A/F sensors depending on whether it was a Federal or CA car. Irregardless, they required different computers and if you had the wrong one, everything was screwed up.
2. Exhaust leak, usually easy to fix. But can add up. Everytime I work on an exhaust system -- some rusted out part breaks. Is this something you want to deal with?
3. Plugs fouling every couple of days. I'm reminded of a Toyota Truck that kept showing up with oil coated plugs. Owner didn't want to spend to send the head out. But it probably needed seals and valve guides.

I'd stay away from this one.

__________________ Note: To reply, replace the word 'spam' embedded in return address with 'mail'. N38.6 W121.4