This i can answer
Just a look on redbook will tell you an R33 GTR will hold its value - and has done, for a car thats between 11-15 years old. However you generally will not find one within the price bracket that youre assigning yourself. Now this may just be in Victoria, but you generally won't find one (in good nick) for less than 24-28k. Do they hold their value? oh my yes.
Is it worth the extra cash?
Most definitely.
Ultimately the choice is up to you. You can go a S2 GTS-t. theyre around 10-15k or so? then you have money leftover for running costs.
A GT-T will soak up the full 20k easily.
Its mostly a choice of what you really want - and if thats a GT-R, then go for it. But it will cost you more, as theyve said, in every aspect.
I'm a 21 year old female, for example. My GT-R is costing me 3100 p.a. on insurance. 60 dollars a week (120 a fortnight) on fuel (silly me runs it as a daily at the moment) plus all the parts for them are that bit more expensive. Spend some time on the forums, do your homework, then make a decision.
And as a side note - you will be hard pressed to find a stock GTR...