None of us have seen it , so we can only base our opinions on the text description. I give my opinion which may not be the same as yours, its the way forums work.
Opinions are based on (edit; your interpretation of) facts normally. “The van is rot and needs £10k spent on it” is not opinion.
If they were moving to Essex I’d have suggested it be racked out to accommodate their collection of white trainers.
If you do the man cave conversion, make the supports high, then you could always make space for a car repair tent around it in a few years time, buy a welder and start rebuilding .. It could cost you a lot less than £10k to fix it, just make sure that at any time you can put the wheels back on and roll it out either so you can call on a specialist for a part of the work.. or get rid of it.
Are you attached to this van it? Do you want a hobby of fixing it? If not you will spend several £k paying someone to fix it ? Do you just want to go camping in a van. Sell it and buy something else.