It kind of depends on what you will be doing. If you will be doing a lot of quality renders, you may need the higher spec PC's. I don't do much rendering, mostly wire frame and hidden line 3D.
I am running 2015 Pro-Plat. At work my PC has an i3-2130 @3.4GHz, 4G ram, Win7 Pro 64, no video card. Not exactly a high spec PC. I can do every thing I need, even use REDSdk for speeding up hidden line viewing. My home PC is an i7-3770 @ 3.4Ghz, 16G ram, Win7 Pro 64, GTX 660 video card. And to tell you the truth, for what I do there is little performance difference between the two. But like I stated, I don't do much quality renders, and when I do it is with LW.
One note, the 2015 TC's don't have the Lightworks plug in, that has to be purchased separately. So if you will be doing quality renders, you may need the LW plugin, or a decent video card. From my understanding, Lightworks relies solely on the CPU for renders, RED Sdk relies on the GPU and CPU for renders. That GPU doesn't have to be a discrete video card. Most Intel CPU's (except the I-5xxx series) have integrated graphics, but you will get better performance when using a video card for RED quality renders. (The I-5xxx series forgoes the integrated graphics in favor of two additional cores)
All in all, I think that just about any new PC would be fine, as long as quality renders are not a main stay of your work.
TC 21 Deluxe, TC 20 Platinum, TC 2015 Platinum, TC 2016 Platinum, TC 2017 Platinum
i7- 3770 Ivy Bridge 3.4 GHz, 16G Crucial Ballistic, ASRock Extreme 4, EVGA 1060 SSC, Windows 10 Pro 64 Bit