Duncan.. I wouldn't worry too much about the new car just yet, the lay down gearbox won't make much difference to the average club driver.....practise is more important..!
The C4.2 is still a good car, the problem being rear end spares, gearbox, shock tower etc...get some spares and it will be fine for some time.
So, like Andrew said, get some practise, wait and see about the new car, then possibly upgrade for yourself and pass the B5m to your son..If a new "B6" comes out , there will, probably be plenty of cheap B5m's about to get a second one for your son. It's always sensible to run two cars the same, for set up and spares.
Have fun...