Having been in the charter industry for over 3 decades, I can assure you a 40 foot monohull is best suited for a day sailing charter boat, not a Term Charter boat where guests require overnight accommodations and 3 hearty meals a day plus snacks, added to the objective of SAILING for a few hours every day, plus non-sailing activities..... snorkeling, hiking, excursions to museums or other shoreside interest, and so forth. No charter guest is content to stay on board your boat hour after hour, day after day........they need to get OFF the boat, and YOU need them to get off the boat so you can get something DONE....like maintenance, varnish, painting, unclogging the head, refilling water tanks and picking up more ice, and getting the laundry done.
Of course, If you intend to offer a "bare bones" kind of sailing experience, then you can only charge a "bare bones" charter fee.....and you will find it virtually impossible to survive on that. Do some research, and you will soon realize that there are only a handful of monhull charterboats under 60 feet that are currently able to make any reasonable living at TERM CHARTERS. I raised a family while living on board a 40 footer, but that is quite different from offering charters! The "cruising" lifestyle does not come close to the "charter experience". Just imagine how quickly your "chef" will tire of cooking for 6 persons three meals a day, using a 2 burner stove and a 5 cubic foot ice box, while still helping you sail that boat for 4 hours a day, and relying entirely on YOU! Passengers tend to be the worst of all sailing helpers......unreliable, and more likely to be injured than helpful.
The best advise you will ever get is to charter a boat yourself......see how it is done, what it really entails......and then determine if you, your boat and your crew are truly up to the job.
If you are not willing to invest in a vessel that is truly up to the job of meeting the expectations of today's charter clients, you will quickly go broke or end up "on the hard" in some boatyard, following a dose of reality and/or divorce. Or both. Sorry, but there are LOTS of 40 footers rotting away in boatyards.......the remains of someone elses "dreams" of being a charter skipper.