Very difficult to give an answer.It is dependent on a great many factors eg the
area and whether that area is a properly licensed one so far as taxis are concerned.
I would suggest that he speaks to drivers in the area he is thinking of ,to get more than one person's opinion.
In properly licensed areas the type of vehicle is stipulated and this can cost well over £30,000. He may also have to purchase a hackney license for the taxi,but he should be very careful here as many local authorities are contemplating deregulating,and the amount he paid for the hackney license would then be worth nothing if he tried to sell it on, at some time in the future.
My advice would be to be very circumspect at the present time,particularly whilst the depression is with us.
Anyway good luck to him whatever he decides.