Insurance does go on the diserability to be stolen or modified, and how many claims it has had in the age group when you insure.
Im 20 years old, been driving since september last year, my insurance for the supra was £1600 fully comp, bit high you might say but considering a MKIII Golf GTI was the same. It's all about the insurance risk of the car, the car is a UK, not an import. Another factor you have to think about when purchasing, yes the imports are cheaper, usually come with more bits to play with but the insurance is a stinger.
Oh and while on the insurance factor, a R5 GTTurbo was £3499 fully comp for a 1.4 turd!!
It all depends on how much you are willing to spend, not in your wallet but in your heart. I'd say never compromise, if you have a car now and cant afford it now, keep the car you have and save up so that when you do want to buy a supra you're in a better financial situation, nothing has to be NOW NOW NOW. And with a supra, finding one is bloody hard. They're are the ones out there that you look at and think, ooh i like that you think i can get that for £11,000. It's only when people are in financial difficulty that you get truly fantastic cars for rock-bottom prices.
Keep on looking, don't go for the 300SX. Very lump cars with a twitchy back-end and only pushing 280bhp when it came out the factory. RX-7's are lovely cars and a true delight to drive, small and very nifty cars, used mostly in the drift scene but again a nice car to drive.
For an RX-7 it depends on exactly what one you get, like most performance cars, especially the supra the big thing is to go single. You'll see alot of single RX-7s on Pistonheads and Gaybay for less than £10,000 but the mileage might be high, you pay for what you get. If UK TT Manuals were less than £10,000 with very good mileage, this club would be called the MKIV Manual UK TT Owners Club....
Oh and for the RX-7, the fact about it being an import also hinders the fact of cheaper insurance for those younger!
That's my 2c, back to work i think the boss is watching