[Volvorama]

For a good used engine you need to either hear/see it running or have great trust in the seller. In your position I'd look for a reasonably priced used engine but assume that it does need some work. If the bores are ok I'd fit new piston rings and shell bearings plus new seals on the oil pump (check this for wear too). A modified rear crank seal would be a good idea, ie a casing that uses a rubber seal rather than the felt ring. This list covers all the stuff that needs to be done with the engine out and things like the camshaft can be done later if required.
I'm assuming that you don't want to spend huge amounts and that swapping engines is not something you want to do often!