1. Very, if looked after, though some cowboy garages have tarnished the reputation.
2. Personal preferance.
3. You can pick up a nice one for £5k or a very nice one for £10k, for the money your 500 will go for you will have your pick of any Supra so it depends how much you want to put into an old Japanese car. You can afford to be very choosey.
4. Imo there's too many wrecks and chaved up shitheaps for the car to be considered a future classic but UK 6spds and the VVTi 6spds will be contendors for the classic crown.
Re BHP vs Reliability- Depending WHO has done what more than WHAT has been done by who.
5. Personal preferance, both have their fans, the auto is very good for an auto box but nobody rips out a 6spd box for an OEM autobox but plenty go the other way.
6. True imo.
7. With respect, any pre Tamora TVR is left wanting (straight from the crate). The Supra has better control, balance, agility AND stability but the Griff has more feedback through the steering wheel. You will feel the weight of the Supra though, it's a much heavier car.
8. Parts are more expensive but you'll replace them less often, most servicing should be little more than fluids and filters.
9. Buying guide is in one of the tech forums
Btw, A chap called Geoff something had a Griff 520 and a Tiptronic Supra, as soon as another member reminds me of his last name I'll link you his PH profile and you can pick is brains, he's as sound as a pound and tells it straight with no bias.