I've done a few builds for Archicad work.
The rendering is done in CPU as the above poster pointed out
The best one at the best price is really something like a pulled (from workstation) xeon 2690 - 12 cores for under 500 USD on ebay - core numbers trump core speed every time. Minimum would be 6, 8 is ok and 12 is excellent!
X99 is really the only choice for expanding it later and the Asus cheapest one is fine.
For GPU megabytes is important for moving around the model if it's big - the number of triangles you can fit into GPU before it dumps into normal memory is critical.
I have an R9 390 with 8Gb mem and an R9 nano with 4gb. The nano goes way faster until I get to a big building, then the 390x trumps it. So it really depends on the size of your models.
I have one workstation with an M.2 drive and another with an SSD. Honestly I can't tell the difference in CAD. Hard drives however are pointless, so go with the biggest SSD you can afford and use it with cache enabled.
I use 1/2 terabyte then shift my non current projects to NAS. You need to factor in something like a WD mycloud.
Get 32Gb memory if you run multiple instances otherwise 16 is enough as the above poster pointed out
Get the best cooler you can afford. Custom watercooling is best but it's like a beautiful woman - high maintenance. A Noctua D15 is the next best choice. Don't use a crap cooler whatever you do. Use a case that can fit eatx and have good fans - Noctua pumps the air and are quiet. They just look bad. I've gone back to them after trying just about every other one.
I hope this helps you and maybe other CAD people
cheers from down under.