What worries me is - is a blown head gasket the beginning of this boat's problems or the end? The last buyer walked away after the survey, and I assume he/she was also offered the same cut in the price.
If you can get a copy of the survey, do so- that will answer a whole load of unknowns.
There are many many boats in the same price range and same LOA range that are ready to go today in your area. Place a wanted ad on craigslist and see what comes out of the woodwork.
All that being said, if it is simply a blown head gasket (evidenced by being hard to start but starting and running, with low compression, no compression, white smoke- if it is hard to start but runs, with black smoke, the problem is rings, valves, or valve seals) It is a relatively simple fix on a volvo diesel- certainly a lot simpler than replacing the head gasket on a DOHC gasoline automotive engine. If you can build Ikea furniture, you can apparently read a manual, so you should be able to fix a head gasket.... and it is a good idea to learn to fix a head gasket.
Replacing a head gasket is certainly cheaper and simpler and less time consuming than installing an outboard mount and outboard and remote controls to the cockpit and a new gas tank, etc., etc...