I'm sorry; I'm still hung up on the fact that this guy is, "stuck with a G Wagon and an E63". You poor thing.
It is true, though... since becoming legal in the US, Skyline prices have become outrageous. Us "AmErIcAnS wItH lImItEd KnOwLeDgE" now have access to the cars we idolized as kids, and are willing to pay outrageous prices for these tired old Datsuns. Truth be told, I paid around 19k for my R33 25T. Best bang for the buck? Abso-f**king-lutely not. Are there faster, cheaper, more reliable, more fun cars out there? Sure. For me, personally, I don't really care; the Skyline has "Zazz", for lack of a better word. Am I delusional? Probably.
Since I'm all about giving unwarranted advice, I would suggest that if you can't convince your dad to allow to you buy the skyline, maybe set your sights a little "lower". For instance, an early C5 Corvette (97-98) has 345hp, is lighter, handles better, and is honestly more fun than a stock-ish Skyline. These can be had for nearly half what they're asking for said Skyline.
Just saying... for $20k, you can certainly do better.