From my vast experience of owning one charter boat in the BVI I'd say forget about chartering an older boat. There are so many new and nearly new boats in the BVI that people just don't charter old boats unless the price is so low it is not worth your while.

My boat was far and away TMMs busiest boat in the first few years but had dropped down quite a way when we left charter at a bit over 6 years old. We then did crewed charter for a couple of years, which worked very well, then sailed around the world, then came back to the BVI and put the boat with what was then termed a 3rd tier charter company. Bookings were poor, costs were high, revenue was low so we left charter, went for one last long distance cruise and then sold.

So I think you should probably forget about charter in the BVI. Things are different -or were different - if you went to other locations, perhaps the Grenadines, although that may have changed now another 6 years down the line.

On doing upgrades to sell. You won't get your money back but they will make the boat easier to sell. You mention custom cushions. You can get these made in the BVI by Doyle Sails and probably others. We had a couple of sets made during our ownership and they excellent.

Good luck with whatever you choose to do.