You haven't specified if you prefer one particular model of Skyline, but if you like R32s you might also want to keep an eye out for the GTS25, which is a non-turbo RB25DE. It has the advantage of a bit more torque than the RB20DE in the R32 GTS, and is still P-plate friendly.
Otherwise, if you're happy to spend the whole 15K on just the car, and wait a while to save up for other mods, then I'd personally go the R34 GT.