I thought I was pretty specific? But ok more details!

All used gear. A dell 5810 or similar (x99 motherboard, 2011-3 socket) - should cost well under $100 with no ram or hd, upgrade the CPU to something with lots of cores (e5-26xx v4) for ~$50 (check ebay if there's none local). Buy at least 64gb of ddr4 registered ecc ram - I’d shoot for 96gb - if you pay more than $50 for 16gb you're over-paying. Workstations like this usually have 6 or 8 ram slots so 616 is a great config. If you go up to 256gb, I think it unlocks running deepseek, but I've not looked into that much.

Then the GPU is the hard part. A $100 1070ti or similar with 8gb is the very least thing you can put in and get usable performance. Barely* usable. 10gb, 12gb or 16gb vram is much better. You can also double up - two of the same make & generation gpu to double the vram. You can mix generations, but you have to be careful as there are incompatible combinations. I know AMD GPUs can work, but I have no experience with them so I have no advice to give on that front except don't mix nvidia gpus with amd gpus.

Pascal generation gpus (like the 1070ti) don’t have much computing power so prompt processing suffers. Newer is better, more VRAM is better. Server GPUs are usually cheaper (e.g. the P100) but require screwing around with 3D printed fan shrouds for cooling, and they have terrible idle power consumption.

If you want to spend more, you can get a lot more capable machine - but then you're off my map. Personally if I had more to spend I would set up this exact machine and then put all the extra money into a better GPU. Unless you have money to burn I would avoid any system that uses ddr5, since the ram cost is a big jump up.