Whether a mining rig will be good for folding will depend on whether folding has high bandwidth requirements.
Typically, a mining rig has many PCIe x1 slots that are connected by a set of risers to x16 graphics cards, but with only x1 bandwidth. So each graphics card gets only 1/16 the bandwidth it would normally get. However, this is fine for mining because there is not much data to transfer between the motherboard and the graphics card. It is computationally intensive, but not bandwidth intensive.
So the question is whether folding is similar to mining in that regard or not.
My suspicion is that folding might indeed be like mining. The graphics card will just receive the molecular formula of the protein, and then it will have to calculate how that protein folds. So my guess is that the amount of data to transfer is small, and that the folding algorithm is computationally intensive but not bandwidth intensive. But that's only my guess.
One way to test this would be to buy a x1-to-x16 riser like this one
and benchmark the folding on it. If folding is almost as fast using the riser as it is using a full x16 slot, then a mining rig would be perfect for you.