How old we talking? I personally wouldn't go further back than 2000 series rtx. A friend has had good luck with Intel GPUs for 'cheap'.

No, you absolutely cannot scale horizontally for speed. VRAM is king, with local RAM being swappable with major speed penalties. SSD is even slower than that and all those are orders of magnitude faster than ant Ethernet you'll be connecting boxes together with. That's not to say clustering isn't an option, just that speed is going to be worse the more you scale out like that.