It all comes down to budget.
Also if you are planning on going the Iray route (preferred by a number of Daz PA's) or 3Delight.
The 1080 Ti is an awesome choice, due to it's 11 GB of onboard memory, and it's speed. You can pay almost twice as much for the Titan XP, but you won't see that much of a performance boost (you'd be better off with dual 1080 Ti's). Of course, hopefully the Titan XP's are built to better withstand the abuse of rendering than the regular 1080 Ti's but I haven't come across any real world case evidence for this yet.
If you are running windows 10, keep in mind that you'll lose about 18% of your VRAM to the OS when using Iray. So the bigger cards have a larger amount of VRAM that is 'lost' to the OS... people have been complaining about this to Microsoft (a set amount would make more sense), but so far Microsoft has been blowing this off...
As for CPU, AMD Threadripper is awesome for the value, but of course if you can afford it the i9-7980 XE is a nice choice. Don't sweat running GPU cards at x8 vs x16, multiple benchmarks have shown that currently x16 only gains you a couple of percentage points over x8, so the gain you get from the multiple cards easily offsets that minor loss.
Threadripper has 60 PCIe lanes available (so x16/x8/x16/x8), but the i9-7980 can do just fine with it's 44 PCIe lanes (4 cards at x8 each, with maybe one at x16).
The trick is to find a motherboard that can support 4 (double width) graphics cards, especially for the X299. And pay attention to any 'headers' which might be next to a PCIe slot, making them 'unavailable' if a double width GPU card is in that slot.
2 GPUs gains you just shy of a 50% reduction in rendering speed. 3 cards will drop it almost to a third, and of course 4 cards will drop it to about a quarter AS LONG AS the scene can fit inside the GPU memory for EACH card (i.e. the memory doesn't 'stack', the scene has to fit into each card involved). So it's diminishing returns after the second GPU.
The high end 'pro' GPU cards are interesting, but of course cost serious bucks...
Having a 'spare' GPU to run the monitor while you are rendering is good advice. The onboard IGP (if your CPU has one) can do this too, as long as your bios allows it (most should, but can't hurt to double check).
The number of cores in your CPU normally won't really come into play UNLESS the scene can't fit inside of graphics memory, at which point the more cores the better! Or if you are using 3Delight, which relies on CPU cores.
A good SSD or NVMe drive is probably a good choice for the 'OS' disk, but also get a large HDD to store stuff on. 4 TB+ HDD's are pretty cheap these days. 64 GB of memory should be more than enough (some would argue that 32 GB is plenty). I'm running an 8 GB ramdisk currently, and have a few of my Daz cache files in the ram disk, which seems to help with performance by a small but very noticeable bit...
Finally, SERIOUSLY look at water cooling options for your CPU and GPU. Rendering is pretty hard on system components, so it's worth the extra protection. Also, overclocking is NOT recommended on rendering systems.
There, hopefully that should get this discussion rolling.
