I have absolutely no wish to p8ss on your parade..................but:-
You will have an unknown van, unless you REALLY know its history
I looked at this, you will need the following, as a minimum:
Under slung fresh water tank
Under slung waste tank
Under slung gas tank
Water pump
Plumbing
some form of heating system for both water and air (Truma 4E for example)
ALL furniture
All mattresses, cushions etc etc
A toilet, and, if poss, a shower
A shower tray
All cooking and refrigeration..................
I could go on and on and on..........the list is endless
I actually did the sums and the above, NOT incl the van, comes to circa £18000, and that's with MY labour
the van, if worth having, will be circa £20000, now, without going through all of the bolleaux, you could simply buy a 6 or 7 yr old PVC for circa £30000, all done and no bother, I just have!! So save £8000 and just buy a used one!!!
Worse though, is that, no matter how well done, a private van conversion is always just a diy project, with low resale value, look at ebay and you'll see what I mean
Actually, I still would like to do my own one, but as soon as I opened the doors on the current one, I went "there's no f'in way I could compete with that!!!!
:have fun: