Most important - Move to Poser 13 - it will really cut your render times down because it uses multiple GPUs for rendering.

Always go with last generation tech.  It will give you the best bang for the buck. In my recommendations below, you save 50% of the cost, but only lose about 20% in performance over current generation - added bonus, you aren't a guinea pig.

I would recommend the following if you are DiYing it:

CPU: AMD 5700x ($180) - 8 cores, 16 threads - enough to run many poorly coded apps (cough, Adobe, Daz Studio, cough) at the same time.  More cores & threads cost more, but don't provide that much more performance.  I have this CPU and have been very pleased with it.

MB: Any X-570 motherboard (starting at $199 new) - you want two x16 slots.

HD1 - 1 2tb NVMe ($66); A PCIe 3 is fine, a PCIe 4 version is about 50% more, but you won't see 50% more performance.  (Mine is a Silicon Power)

HD2 - 1 4tb SSD SATA drive ($140) - you will need this for space. I have eight 2tb SATA SSDs - Poser 1, Poser 2, Daz Studio 1, Daz Studio 2, Stable Diffusion, and 1 spare. I went with multiple 2tb drives (and a PCIe Sata card) when the 4 tb drives had a $50 premium over two 2tb drives - the 4tb premium is now less than $10, and you won't need to purchase PCIe sata card, the X570 will have enough SATA ports.

Video card - Two RTX 3060 12Gb ($262 each);  - Opitx will use both, and it will cut your render times by about 50%.  Each of these have  over 3,500 CUDA Cores. These cards will be available for years - Nvidia is sitting on around 6 million low end Ampere cards. If you are willing to look at used cards - RTX 2080ti - same price as a new RTX 3060, but roughly 25% more CUDA cores.

Don't waste money on the RTX 40 series - the only one that provides a boost over the prior generation is the RTX 4090 & it starts at $1,600.

PSU - get a 750 watt 80 plus gold rated ($110 or so).  Don't skimp here.  A PSU that lets the magic smoke out will usually take other parts with it.

Case - A case is a box to hold your parts.  Avoid RGB.

Windows 10 - get an OEM key - run you about $15.

Total cost is about 1300 before tax, and will take between 30 minutes to an hour to assemble with a Philips head screwdriver.