I repaired my engine with new rings and a de-glaze. But I only did that because I am not allowed to do engine swaps in Spain. The thing is, if you buy a second hand bottom end, you don't know the condition its in. Even if you get one from the scrapyard with a warranty, the moment you take off the head, the warranty is gone.
Look out for a second hand engine from a crashed vehicle. Then you know at least that the car was not scrapped because of the engine! Or wait till you find one in the small adds from a car that is being broken, with the engine still installed. You can then listen to it and even do a comp test on it. You can at least make a good assumption before you buy it.
The cost of a mech to do it, I have no idea, but the job to do it yourself seems daunting, but its not as bad as you think. The read the manual...carefully and ask plenty of Q's!
