Out of curiosity - buying another CPU fan and plug it in should be like $10... Unless your system is ANCIENT and you plan on upgrading anyway, why not just fix your old machine?

I have 2 PCs that I use - one is an i7 2600k that I built 6+ years ago - it's still plenty fast for just about everything. I use it for server/media duty nowadays at home. I've recently built a compute monster out of "older" parts (mostly for work, a little bit of play, and a little bit of hobby (poser) :) ). Threadripper 1950x, 64GB RAM, 2x Titan XP (with a 3rd one on its way!). Building it was an experience in itself due to the thermal characteristics of the components. Some observations:

If your primary workload is CPU, or do mostly CPU rendering, then get something with a lot of threads - I recommend the newer AMD Ryzens for sweet spot in price and performance. I don't recommend Threadripper because unless you have specific use cases that benefits from TR4's PCI lanes, you can get 95% of the performance for less than 50% of total cost.

If your primary workload is GPU - and you are using Poser Superfly - you are limited to the 1xxx Nvidia Pascal GPUs. I picked Titan XPs due to their 12GB of RAM. However I also don't recommend Titan XP because the default cooling solution that came with the card is terrible and doesn't do the GPU justice. The best deals would be a used 1080Ti part - you get 95% of the performance of a Titan, only 1GB less memory.

Cooling is not a big deal in most use cases if you are just going with 1 GPU. If you start cramming 2, 3 GPUs in a case then you need to have 1) a high quality PSU and 2) a good cooling solution. Either lots of fans or straight up watercooling for the hotter components.