The Radeon 5700s are nice cards, but brand new, so driver support in various programs may need to go through a teething process. Not a big deal, but depending on how slow the company that makes your software of choice is, well definitely check their customer support forums and such for people trying the new cards.
You mentioned Daz Iray though, so that pretty much makes the choice for you. That being a RTX 2070. The new version of Daz officially supports the new cards, and for Iray CUDA rendering, you pretty much are limited to Nvidia.
Since CUDA also helps with Blender cycles, well there ya go!
As for CPU, for rendering purposes an AMD 400 series motherboard (B450, X470, etc.) with the appropriate BIOS update will do you just fine. Someone recently benched Daz Iray at PCIe x1 vs x16, and saw very little difference in rendering times. Once the scene is loaded into the graphics card memory, the calculations are done pretty much entirely on the card, so PCIe lane speed isn't that big of a deal,
Don't sweat the whole RTX thing. It doesn't help all that much with rendering, so if you find a deal on a 1080 Ti for a similar price to a 2070, yeah more VRAM good...
The 500 series motherboards bring other things to the table, the main thing being more 'onboard' PCIe lanes depending on which board you pick. The faster NVME speeds can translate into faster boot times and a few other tasks being a bit faster, but for me, the big selling points are the boards that have 3 or 4 full length PCIe slots.
Threadrippers are now a bit cheaper thanks to the launch of the 3900X 12 core AM4, and the impending launch of the 3950X in September. Threadripper's advantages are more memory channels and memory capacity, and 60+4 PCIe lanes and 3-4 full length slots for graphics cards.
The CPU will matter if you do a lot of CPU based rendering, but if you can do GPU based rendering, and fit your scenes inside of the graphics card VRAM, well the CPU isn't of much help at that point. The 2070 has 8 GB of graphics memory, so it's workable, although the 11GB 1080 Ti would give you a little more headroom. The 2070 might be as fast or a bit faster, but either will beat CPU based rendering any day of the week, for programs that can take advantage of GPUs.
I'd probably look at the boards first, and try to find one with 3 or 4 full length 'double spaced' PCIe slots. Daz allows GPU based rendering with multiple GPUs, which cuts down on render times considerably. Then, I'd pick at least an 8 core CPU. Really, the 2700X should do you just fine, although the 3700X is a bit faster at other tasks, and also for CPU based rendering.
AMD has demonstrated with the 3000 series Ryzens that the CPUs will play nice with a number of older boards, once you update the BIOSes, and the kinks are getting worked out in the newer bioses now. The reason I mention this is that since the EPYC 7nm server CPUs are supposedly backwards compatible with the existing EPYC boards, and the AM4 chips are as well, well there's a good chance that Threadripper will continue this trend on the TR4 socket. So if you grabbed a cheap Threadripper system now, you could always buy a 7nm Threadripper CPU later if you wanted something faster at that point.
Following that thought, you could grab a cheap 1900X 8 core Threadripper chip for now (they are less than $300 at the moment), and then grab a new 7nm Threadripper down the road, using the same motherboard. This might give you more wiggle room on your GPU budget.
Another strategy would be to grab say a cheap B450 board with at least two full length PCIe slots, and grab whichever Ryzen appeals to you, with the idea that you might drop in the 16 core 3950X at a future date. Or one of the cheaper X570 boards, although they are a bit more pricey.
Also, try to get 32 GB of ram. It helps. I'd recommend two sticks of 16 GB each, with reasonably fast timings. That way, if you want to add more memory later, you can just fill the unused memory slots, assuming your board has 4 or more memory slots.
Storage is cheap these days, and you can always add more later. I'd recommend installing your Daz install on a separate drive or drive partition, as the Windows Update installer has occasionally fucked with Daz installs... 2 TB of storage served me for quite a while, although I've now outgrown that thanks to all of my Daz assets and scene + render files.
Some programs do CPU based rendering only, at which point more cores is helpful, so keep that in mind.
The 3700X and 3900X are awesome for CPU based rendering, but still MUCH slower than GPU based rendering. Sometimes, though, you will still need to do CPU based rendering though, so at that point it might matter more. Hopefully, though, at that point you can afford to upgrade to a better CPU. But GPU based rendering will give you the most bang for the buck.
Anyways, to summarize, since Blender and Daz both can take advantage of CUDA based rendering, grab the Nvidia card with the most VRAM that you can afford, then build the rest of your system from there. I'd recommend picking a motherboard with multiple full length PCIe slots, so that you can add more GPUs later for faster rendering as your budget allows. Try to build your system with future upgradeability in mind, as far as having a few spare PCIe slots and such...