Have you used or are you familiar with any CAD software?
Do you intend to use web based CAD (such as TinkerCAD, Fusion 360) or an Offline CAD program?
For me a large screen is *extremely* helpful. OS... Windows/MAC/Linux is a personal choice... i.e., use what you are used to using. Processing power is helpful the more RAM/CPU/GPU power you have the better (I use a desktop with an overclocked cpu, overclocked RAM & RAID setup SSD drives... it's quite perky).
Most of this is personal preference...