I can give you some insight and or things to consider. So running from St Thomas is doable. You will need a hundred ton masters with sail endorsement and an SCTW 95 safety training. As far as owner operated that is actually the normal scenario for the type of business that your talking about.
You need to look long and hard at what your goals are in this. It is an expensive business with low returns on investment. Most people do it to offset some of their costs for a cruising life style.
Things to consider, do you have the capitol available to buy a boat outright? Financing can and will be an issue on many levels. Also do you have the resources to carry the cost for several years in the red. It will take several years to build your reputation with the charter brokers and build up your business. Also the costs of maintenance can and will be significant. Annual haul out for hurricane season, this bill alone on cat of that size will be pushing the better part of ten grand. Think a thousand a month for storage three or four month minimum, paint, buff, wax, anodes, sail drive service, etc.
Then factor in the oh sh!! factor. Things like the generator just died I need a new back end fed exed from Florida and have to pay how much to make my next charter? I work in the bare boat industry and I can tell you these things happen. The age of the boats that you are looking at are a factor as well. Upgrades to systems and equipment will be an ongoing and lifelong affair. In the tropics air conditioners last about five to seven years, batteries last two to four years, sails are good for five to seven under charter use in the trades, as do biminis and awnings. Mattresses last about five as do most soft interior surfaces, settees etc. Then you add in water pumps that constantly break, heads that clog, alternators that fail, etc etc. I would budget twenty grand a year for repairs and upgrades on a boat of that size and vintage.
Now not to sound like a negative nilly. If you think you are cut out for all of this it can be very rewarding lifestyle, it won't make you rich but you can live like a line from a Jimmy Buffet song most days.