I think your question has been pretty well covered by the other thread.
Re Tax no the road tax does not go down with the age of the car. It is based on emissions and will vary between £90 and £160 for a year - either way not a big cost.
As I said before Insurance could be the big cost item and it is worth getting a certificate from their USA insurance company (Stating so many years claims free) to ensure a reasonable price is obtained
Most cars in the UK are manual and a lot smaller than those in USA - worth checking if they can drive manual or are on a restricted (Automatic only) ldrivers licence as automatics are fairly rare in smaller cars and actually normally come at a premium in the UK
The thought of leasing a car may prove an easy route as thay will not have the problem having to dispose of the car at the end of their stay and moving a car in a hurry is always costly!
For ideas of lease costs see http://www.jamjar.com/content/leasing/
These may not be the best prices and so they should shop around. Oh yes VAT is normally charged on top of quoted prices and will add 17.5% to the bill.
As for the job sorry cannot help on that but advise him to visit a Department of Social Services office and get a National Insrance number as most employers will lose interest without one of these!
Have your friend e mail me if he has any other questions