If you are ordering from V1, buy the kit and then add your extras. If you add additional timing belt at the same time as your kit , Ryan will just increase the length of belt he ships. He will ship a single length, not cut it for the extra. As for the “harness,” just add additional extension cables. You probably want to order five of them, one for each stepper motor.
I do have some concerns about your build. In terms of size, a 4’ by 4’ working area is at the outside edge of what I’ve seen on the forum for successful Primo builds. Many on the forum would encourage you to consider building a Lowrider instead for a build that size. As for the tubing, steel bearings on aluminum tubing, even with an anodized surface, is problematic in my mind. I cannot say it won’t work, but I’d have a backup plan. Or maybe start with a single pipe test piece and one of the trucks, and run the truck up and down the pipe for some time to see if there is an impact.
As for plasma, you may want to spend a bit of time researching on the forum and elsewhere before you place your order. I’ve just skimmed the topics, but there seems to be some special considerations when making an MPCNC into a plasma cutter. Getting the right torch and dealing with the electronic noise are two issues I’ve seen mentioned, and I’m sure there are others.