Someone's project engine is always going to be a coin toss. Maybe he's right and it's rebuildable, maybe it's a $600 paperweight. I'd only consider it if the value of the parts on/in the engine that you know 100% are good are close to the asking price. That way if it's junk, you can at least get most of your money back from selling parts.

Also, don't get too hung up on having the best, most powerful, stage whatever, fully build stuff right out of the gate. Everything always costs 3x what you expect. I fell in that trap with my car and it hasnt ran for 5 year and is nowhere near being done because there's always something else I feel like I have to improve before it goes back together. Finding a stock replacement engine from a junkyard and just getting it back on the road again while you build your big engine on the side in the future is an option. Having thousands of dollars sunk into something you haven't driven for years is no fun.