Hi, Sorry if you’ve already reached the conclusion but the inboard diesel/generator/heater is going to be the big constraint. I can comment on some boats I’ve sailed that might be of interest. There are a few nice 20ft boats of the centre-plate, water ballast variety with an outboard in a cockpit well. I have good experience of a friend’s Hawk 20 – we have beached it and pottered through salt marshes. The cabin version has a pretty small cabin but it can sleep two 6 footers. There are similar boats with a smaller cockpit and slightly larger cabin but the hawk sails well.
The inboard diesel requirement probably means something heftier and traditional – Cornish Shrimpers (19ft plus a bowsprit) sometimes have an inboard Yanmar engine fitted. I have crewed in the outboard version and the cabin is big for a 19ft boat. Lovely gaff rigged boat, so nice easy short mast designed to drop into a crutch – but need deep pockets. I have sailed a Drascombe and they handle easily with a choice of sail plans. I’d go for the Drascombe or Hawk types, and use an outboard and portable heater (while awake only) with a CO detector, plus LED battery lighting. There are ways round other power requirements. Trouble is it only takes a relatively cheap little 4 to 8 hp outboard motor to push a 20 footer at hull speed, and you can lift it off for servicing. In the Hawk and others this can be tilted and the recess sealed with a hatch – no drag and allows beaching. Inboard diesels are super reliable and arguably the best by far for larger yachts but expensive to buy and fit, heavy with that water cooling and heating system and require a shaft through the hull and a prop generally permanently in the water. There are a few more options (and cheaper) if you go to 25ft, though the majority of these are still outboard propelled and it’s a LOT more boat to trail.
Try the list on this page http://www.go-sail.co.uk/hometrailer
Good luck with it.