Professionally, well over $500k.
Depends a lot on the internal finish - you could add another $150k for nice timber work. Electronic packages can also get rather expensive.
Work on at least 2500 hrs.
Powered sail handling/ carbon spars etc can add on as well.
Downsides:
1./ It has been "heavily built". This is supposed to be a lightish boat,so you will need a trick lightweight interior to compensate. You will end up with minimal tankage and cruising luxuries
2./ Balsa core is not ideal.
3./ Deck is bonded on. Will have to be cut off to do job properly.
4./ It's a dated design. It was , and still would be quick, but modern boats have much more volume.
With changes , upgrades and cost blowouts, budget on $750k, and you will get a nice cruising boat you may be able to sell for $300k.