I would take a look through the
PC Build Thread to get some idea of what combinations are working for people.
As far as how much computer you need really depends on the size of your sessions, number of plug-ins/VIs and the complexity of the session. It also depends on which plug-ins you use as some take up a lot more resources than others.
For Thunderbolt, I would look for a board that already has it built it. The add in cards seem to cause some issues for people, and there are more boards out there now that already come with TB3.
Before going TB3 though, check with the manufacturer of the interface you will be using. Most interfaces these days still need a TB3 to TB2 adapter and they can tell you which one works best with their hardware.