I have far less pausing using Win8.1 basic version (it came with the LT) than Win7 Ultimate, I run TS12 at higher settings on Win8.1 and routes load faster. The only problem with 8.1 is the display scaling which makes text look blurry, but that is solved by disabling display scaling in compatability mode, and the metro screen but you can download a free program to get rid of a bunch of the apps that you don't use, just look for win8appremover and you will find it. I am thinking of switching my desktop over to 8.1 too if that is any indication. I think those who say they hate 8.1 are those who don't take the time to tweak it and learn it, sort of like reading instructions...why bother? IF you really hate the 8.1 look you can get free software that makes 8.1 look and feel like 7, I tried it but went back to 8.1 default screens.
You will always get the best build, IMO, if you build yourself instead of buying from a big company who are going to try to suck money out of you. If I were you I would put together a prospective rig and then post the specs on several sites like Tom's HW, here, etc, and get an overall impression. Maybe there is a better ram you overlooked for the same price, cheaper GPU that does the same job etc.
Best of luck. My last point is build for tomorrow, not what makes it work today.