[doc_rage]

As long as the individual knows what they're doing and you have confidence in the work/engine, and if it won't take all of the $500, go for it. I don't know what was meant by "for the cost of a lift (3 6x8's for a motor lift)", but I would assume it would be a body lift (~$80), and some sodas, not bad at all.
I just posted a long-winded response about my thoughts on getting a new/rebuilt engine, but same idea would apply towards a used engine. In all honesty, if it were me and I knew the person well enough, I would go ahead and have them do the crankshaft. If the rest of the motor is sound, and that's all that needs to be replaced, bearings and related components included, I would go for it.
Meeh350 raises a really good point though; other components will wear out. Better to get a motor that's been all together for a while. The 2.8s are all over the place, see what it would cost you to get one before you decide to get the crank installed by the friend.

 

[doc_rage]

I wouldn't do it. I went in with the same idea of getting a cheap truck and just doing the motor. Now, I've replaced related engine components, ball joints, brake drums, brake lines, parking cables, wheel hub assembly ($70-$170 apiece normally), and a bunch of other stuff. That's all just to keep it running on the road. I am just now catching up to doing what I want to the truck of my own accord (the body lift and wires, etc) after a year and a half of replacing OEM components. The truck also had over ~200K miles on the body when I got it.
I'd just spend the money on the vehicle you'd be driving the most.