It is a big job but it depends on what you want to do with the van when its finished. At one extreme it might be just for weeked trips in the summer, at the other you might want to have a van capable of independent travel pretty well anywhere in very cold or very hot temperatures. Or somewhere in between. You also need to be aware of weight as it is easy to overload the vehicle. There are certain requirements you need to comply with if you want it registered with the DVLA as a motorcaravan. Gas installations need to comply with regulations and at least must be tested by someone qualified. Also bear in mind the
insurance for self build vans is not easy but not impossible to obtain. It would help if you joined a self build group and perhaps buy
Build Your Own Overland Camper by Steve Wrigglesworth and / or Build your Own Motorcaravan by John Wickersham. - both Haynes manuals.