You really can't estimate until the motor has been torn apart and the inside of cylinders are examined. What is the reason for rebuilding? Low compression? Typically, if the cylinders are still in good shape then it may just need a new set of rings. Not very expensive of a job but then again they may need to be honed out at a shop and larger rings installed. There could be a number of things that are damaged, not just the cylinders, so do not get your hopes up until the motor is opened up. Just hope for the best... :-/
What was the root cause of the low compression? Asking a few questions can give you a better idea as to what might be wrong with it. Those motors do not typically loose compression on their own and it had to be so sort human error. It could go as far as a bad crankshaft and or bearing which would involve a lot more in the tear down process and also cost much more $$. Its hard to say but if he is willing to sell at a decent price then I would probably roll the dice on that one.
Find a company who sells short blocks, and that would be the worst case (plus labor of course); however I don't not recommend messing with it. You can find a Mid to late 90's 15hp that is in really good condition for under $800.00. And I almost can say with certainty your going to be at that or more for a rebuild, after carbs, water pump, etc, etc.
When an engine overheats the rings can lose tension and the compression will be low. If the cylinder walls can be cleaned up with a little honing I would put in a new set of rings and try it. I have brought back more than a few duck hunters engine by doing just that. I haven't tried it with larger than 25 HP, but on a small one it is worth a try. To check the cylinder walls you just remove the head and a new head gasket shouldn't cost much.
I bought a locked up 15, I mean a breaker bar on the flywheel nut, hit it with a hammer and no movement locked up. The factory used too much gasket sealer and it blocked the oil passageway to the lower main seal. I had it honed, got new pistons and rings and broke it in good. Sold it to a friend and he ran the dog-do out of it.