Did you manage to source a van ?
What you are/were/looking for will be very expensive; and that is just the van.
Converters wishing to make a profit on these will be concerned about their margin; hence, imho, they will be rare for some years to come.
Once you have a cost for the van, add at least £5k for a conversion, and then the sale price will include the dreaded non-recoverable (unless you are a business) VAT.
I am aware of a converted 2008 camper close to me, by a very well known high quality converter elsewhere in the country.
Asking price is not below £30k.
I will not even try to suggest what budget you will need, and, of course, even that depends on your requirements.
Good Luck.