15K for a modified 33GTST will be a little hard mate. any modified car you get its best you go over it with a fine tooth comb to make sure your not buying a dudd. and for that budget im sorry to say this but it will be hard trying to find a nice clean well looked after modified car.
being your first R33 why not try looking for a stock R33 series 2 skyline? you can get a nice clean one for around 12-15K and mod it your self.
main thins to look for is the service history of the car, if its been well kept, quality parts used, engine condition. prop best to do a compression test. if you want get the turbo checked out for shaft play at the same time.
but being your first skyline don’t by a modded 33 at that budget as a first car. or increase your budget.