As others have said and assuming you do not purchase a dud. The ongoing maintenance costs will be similar (if not slightly higher due to needing quality oils) to any other Japanese 15-20yo car. If this is your first car, I don't recommend a skyline, great car yes, but not for someone new into cars and who obviously has a strict budget. For the same price you can get a lot newer, cheaper, run about.
Like any car if you decide later to drive the car very hard, do not maintain it correctly, or to modify it to output significantly more power, your maintenance costs will increase.
Common Skyline specific workshops include Trojan motorsports and ESP, however any workshop can do basic maintenance. You could probably get one of those mentioned shops to do a inspection before purchase, may costs a few hundred, but it will likely save you thousands if you avoid a dud.