How and where do I go about getting a valuation for a highly adapted Renault Minibus for disabled use
I doubt if you are going to find a good valuation.
Where is this Renault? How old is it?
I would suggest finding out what the price would be for a new vehicle of like capabilities and adjust from there.
Or price a used Minibus with similar age and condition/miles and add the cost of the adaptations, minus 20-30% for the adaptations not being new.