I think a good looking magnum engine is worth about $200-300 if it inspects ok.
The only advantage you have on an assembled engine is if you know that it runs, meaning you have seen it run or have some sort of guarantee.
I would rather buy a disassembled engine, than one that was put together that I couldn't get some kind of documentation or assurance on, like hearing it run. So for what it's worth (to me), I think the engine is of more interest, because I can mic it and inspect things like bearings, journals, crack prone areas, valve seats, etc.
FWIW, most people have a hard time buying engines because they are together and have no way of knowing what they're buying.