Personally speaking, If I had a van with that kind of mileage, I would not put a lot of money into a conversion. I don't hold with any comments like "bullet proof" or "designed to do 500,000 miles" or whatever. 200,000+ is a big mileage whichever way you look at it.
If it is your first conversion and a bit of a trial run, ok. If you are hoping to make your "forever campervan", start with a less worn out base.
I bought a VW T5 2.5TDi to convert - they are "bullet proof" as everyone knows, eh? Previous owner spent around £10,000 on servicing and repairs in the previous 2 years before I bought it. I spent about £4-£5,000 in servicing and repairs before I sold it 18 months later (to do a new conversion).
That was a 170,000 mile van. I doubt if it was a 70,000 mile van it would have needed to have nigh on £15k of money dropped on it!
You may save a fair chunk of cash on the base vehicle, but the cost of your conversion will be the same be it a high miler like that or a brand new van - and the brand new one will last a lot longer and repay you better (and for brand new, could read "nearly new" or 'reasonable miles').