How much you wanna spend on car? Cause if you got 18000 to spend and u spend 13000 on car plus insurance (guessing young male) and good alarm system $3000 you wont be left with too much.
Whether a car is boosted or stock you cant entirely tell its history so buying any second hand car is a gamble. Do the usual checks when looking at car and be patient!
I would reccommend if your a noob with skylines by a stock one and get to know everything you can about them and their mods. Then when you are ready, do it up and you have a car which is uniquely yours.
Just my Personal opinion, all the best on your skyline journey