Currently I'm using a 13HD Cintiq with a laptop (running on Windows) that I configured myself. I don't know a lot about computer specs, but I did a lot of research and came up with a configuration that seemed alright. Then I posted that plan on an online forum for computer projects/reviews/etc and asked feedback on that plan (and mainly asking then if the setup I had in mind would work for the purpose I needed it for). They provided me with a lot of great feedback! I then had the company I bought the laptop from put it all together. The laptop cost me about €1600 and the Cintiq €600 (2nd hand but in great shape).
If I would have put all those specs into a pc rather than a laptop it would probably have cost me half the price, but I needed to bring it to school with me. It was still a lot more economical than buying a Mac with the same specs; that would've cost me around €2500.
It can also be very rewarding to look for 2nd hand pc's. I regularly see 2nd hand Alienware pc's that are more than capable to run several graphic software programs at the same time for less than €800.
Good luck with your search!