Hi Gary, I would 'spread the word' about the auction on a few car forums so that people realise it is for such a good cause.
The 'trade' value on the 4.0 V8 would be peanuts (even with the low mileage), probably £1500 or less, we recently gave £750 for a reasonable 2000 registered 4.0 V8 that had done about 60k.
It is worth what someone is willing to pay, myself I don't rate the pre-facelift S-Type at all (poor interior, poor gearbox, handbrake that is in the wrong place, etc) but a great car if they are cheap !
Putting my prejudices aside, I would be looking to pay no more than £2500 if it is a really nice example but I am still convinced that it is not a Sport version (not a big deal but sellers who can't describe the vehicle accurately annoy me !)
It would also need to have very good service history to get top money.
You really need to drive the 4.2 version (all 4.2's are the facelift model with 6 speed box and electronic park brake) it is a world apart from the early version.
Will this particular 4.0 be a collectors item one day because of the low mileage, who knows ?