[joshvm]

Yeah it's definitely cheaper long term.

The biggest issue you're going to run into is available RAM. Card speed is less important, you can always train a bit slower, but you can't get around memory limits. A recent *80 series would be fine to start, e.g. the 2080/3080 have 10-11GB. You have to go to the higher end cards like the 3090 to get 24GB. Generally working on a single card is easier than distributed, so one 3090 is better than 2x3080 IMO. Fortunately the mining crash has made stock a bit more available and prices have stabilised a bit.

If you want room to upgrade to 3 GPUs then it's going to be a reasonably sized case. These are 2-slot cards at minimum and you're going to need to either buy a big PSU up front (1kW at least), or upgrade later. Any high end motherboard with enough PCIe slots should be OK though.

I don't have a particular guide in mind, at this point I would say just post on Reddit /r/buildapc or /r/machinelearning and say you want to build a 3-GPU machine, or see what others have built on there. You can use something like PC Part Picker https://pcpartpicker.com/list/ to filter by compatible components.