Desktops will be the same for Windows and Linux, so if you have built one there you are set. It sometimes pays to do a bit of background research as the driver situation can be more painful for some hardware, but these days darn near everything in desktop land "just works".

If you do want to find a modular laptop, look at a company called Clevo. They are an OEM for Sager and a few other high end gaming laptop brands. They are the closet thing to a modular laptop chassis in existence -- though from a cost/performance perspective you will almost certainly be better served by some mass produced consumer laptop. Modularity in portable machines just isn't very economically competitive.