Evening chaps
Cars I've looked at:
Celica GT-Four - Suspension component costs scary, unknown running costs
This is what i've owned for about 1.5yrs now, I was also 21 when I bought it. I will give you some answers to your questions no problem.
First of all, if you live in a decent enough postcode the fact that it is an import should not be an issue, specialists generally won't insure you either until you are 25+
Luckily, Elephant seem to insure anyone with any car for cheap, they don't seem to care.
At 21 my import was ~£900 on insurance, and this year it is £760 with all mods declared.
Your budget is much higher than these cars currently go for due to the economy of not many people wanting a turbo petrol at the moment and as you rightly stated, the sheer cost of some of the suspension components.
Also, is a 4WD turbo something you want? I gather probably yes if you've been mulling over the 3000GT
Circa £3k will get you a good solid car with decently low mileage with all the work done usually, there are numerous ones for sale on the owners' club (GT4OC) and I wouldn't dare buy the car from anywhere else again due to the awkward suspension setup and rarer nature of the car. Plus on the forum you can search the history of the car which is a big plus to see any previous owners complaints etc.
As a bonus you then have cash leftover to continue saving towards something else, or to start modifying it some.
Performance wise the JDM 1994-1999 GT-Four ST205 is what you would want; 255-275Bhp stock/usual filter, exhaust mods. (Same engine as the same years turbo mr2's.)
If you find a car that has the suspension components replaced recently you should be fine as their lifespan is around 60,000mls.
You want things like control arms/banana arms, figure of 8's to be replaced.
Recent change of clutch is about the only other thing that could be costly due to the engine usually needing dropped to get to it.
Turbo's, it's an 18 year old car, it will have had some use if it's a higher mileage car but just check what it's been tuned to, if you find a standard-ish low miles one it should last some time, it isn't paper.
It would also be beneficial to get a car that was imported quite recently, 2008+ perhaps as it will be less likely to rust with the Japs not using road salt, and freshly undersealed upon import.
Running costs: What would you like to know? You said MPG is irrelevant but I can average 28-35mpg driving normally easily, foot to the floor all the way, who knows 18+ ?
It is good practice to change the oil and filter every 3,000 miles, or every 6,000 if you use high performance oil. I just stick to decent priced oil and change every 3k.
Parts are generally available brand new genuine toyota from the likes of TCB, or anything else on the car can be had from the members of the OC.
I bought one with all the receipts for suspension work etc being done and nothing has gone wrong with it over the last 18 months, apart from the chargecooler radiator bursting when it was -16 degrees or so up here in Scotland last winter; but it was the original 18 year old one so stuff happens. Replacement from the OC cost £35 delivered. Doddle.
Please do ask any questions you have, and sorry for the wall of text...the better you know..
HTH.
A few within budget - 
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