If you buy a boat with a pump out toilet I would suggest you "live with it" for a while and consider later whether you really want to get rid of it or not. With cassette toilet you have to carry them to Elsans, which can be very unsavoury places, to empty them regularly (days apart, rather than weeks, I gather) . With composting (or separating ) toilets you need to find a way of composting the poo (not now allowed to put it in C&RT bins) and with a pumpout loo you have to pay to have it emptied which can be anything between 2 and 6 weeks use depending on the size of the tank. I would recommend living what your new boat has for a while before leaping into action to change it.