Any modern gaming laptop will do the job nicely with a higher end dedicated GPU as suggested above. Always run it plugged in.
If you take a joystick, make sure the laptop has free ports otherwise an xbox controller can also work.
If you regularly switch between the desktop and laptop, you can keep in sync by copying the 'community' and 'official store' folders after any updates to save external bandwidth.
If you fly the fenix, you will have to deauthorize it from one machine and reauthorize on the other through their UI. Gee,I hope they release a codewheel in the next update...