If you want to sell it quickly and locally, I tend to agree with the 15K number. If you want to do some work and give it some national exposure via e-bay, Hemmings, or even some of the auctions, you might be able to get it to the 20K - 25K range.
Personally I think the BJ8 engine is a real plus, it is far livelier than the original Bn4, especially the two-port. Most Healey buyers seem to lean more toward performance than originality -- witness all those 72 spoke wheels out there.
Comes down to quality of paint, panel fit, interior. Improving these issues is very expensive, easy to spend 10 or 20K just to upgrade from 20 footer to five footer. Or 100K to take it to concours level.
I am a bit behind the times on Healey values though, seems like they can generate some major bucks amongst collectors, you never know. In the end it is worth whatever your buyer is willing to pay.