Hi mate, I'm 20 (21 in less than a week) and just picked mine up yesterday. You'll love it, so awesome choice of car.
Regarding your question on budget I would say that you'll either need to go for a higher mileage one or you come across the odd Cat D (write off but the best kind if that makes sense...google it!). I don't know much about Cat Ds other than they are much cheaper. There seems to be a lot of talk on the internet about these costing more to insure but when I asked admiral they said it didn't but they needed to know anyway.
The clutch issue from what I can gather seems to vary...for some folk it's given out after 40k miles, other's have reached 100k+ and never been changed.
In terms of whether they're JDM imports or not look out for ones that are "Fairlady Nissan 350"...these ones are imports but again as far as I know there's no major issue with this
The biggest piece of advice I could give is be prepared to walk away. Despite them being far from the most common car around there are plenty available so hold out for the right one for you.
Regarding any other mechanical bits and bobs I'm no expert so I'll leave that to those who are!
Good luck with the search