A votronic mppt regulator will take up to 50v, it also has a 1 amp blead to starter battery, depending on total wattage a 250 will cost about £140, there are also others out there, some a bit cheaper.
Not sure about that, skoda alloy engine decended from ninteen fifties lasts forever if looked after, ford new wet belt fiesta engine might get you round the block on a good day.😁
I have one of these 12v units in the loo/washroom, i switch it on from the cockpit when driving, ok of batteries and solar when parked if only lightly used, gas or diesel best if parked up in wintertime.
I carry 2 5ltr water containers, i fill at home before going away for a day or two, any longer and a filtration filter sys will be required piped from the main tank.