The problem with "older crusiers" is that many can be many unknown brands and designs. So its hard to guesstimate a market value, regardless of condition.
Are you talking about targeting "production brands" or anything?
Ive seen boats re-appear for sale 1 - 4 years later with a totally different description, but a photo that matches 99% the previous time it was forsale. I had a collection of Trade-A-Boat magazines for while.
EG:
28' trawler style GRP cruiser with CAT 3208
$35K, then less than 4 years later it was forsale under $20K apparently in good running order.
22' Riveria Design Sports Cruiser with Mercruiser (looked like 2400 Mustang, Savage Ranger 24 and Signature crossed together with those slanted cabin windows pointy at one end)
$9K then seen it around $20K a couple of years later with no mention of a new power plant or refurb)
Custom 10m Alloy plate aft cabin cruiser. Twin Diesel Shafts.
Seen it for sale several times with all different prices varying by 100%.
Your no idiot, so i know that you will pay the correct money for the boat. But how much is the "new owner" prepared to fork out for it. With unknowns there is nothing to bench mark against.
Id say that you need to stick to the known production brands. Ones that will sell based on brand and model.
If i was buying a refurbished boat, id like to be able to see your workshop or other boats that are work in progress so I could verify the quality of your work. A progressive photo portfolio of work would be good. Especialy for new floors, underfloor tanks etc that are not accessable for the buyer to inspect once installed.
You always tell people to tread carefully buying a boat that appears too clean as it could be hiding something. Now its your turn to be the seller