You need to get a good accountant to evaluate the business paying money for goodwill of customers is all very well but there is no guarantee they will stay.
Seems a little strange if he has an average of 45 tests a week and only wants £40k for everything.
I used to know a guy who bought a similar business to what you are looking at testing about 15 cars a week + repairs paid £35k for the lot, first problem was vosa would not give the station a licence until the bay and entrance had been radically updated which was another £25k. Had numerous other problems with planning, noise,, and parking ect, he had no legal advise or anybody to scrutinise the books objectively he is doing ok now but it came at a hefty lump added onto his mortgage.
Not saying its not a good deal but be very careful get a good accountant and solicitor if it all stacks up give it a go.