From my experience with minecraft (which may be a bit dated) It can be cpu intensive, but faster clocks will help you better than lots of threads. Ram is important because it's java, but 6GB should be sufficient, but 8 would be nice. VRAM is very important for high-res texture packs but I don't think gpu speed itself makes that much of a difference. SSD also provides significant improvements. I imagine you can get away with a pretty small one, but I suppose that's up to your judgement. This can probably be done pretty cheap, especially if you get used parts for a lot of it (and there's no reason why you shouldn't be able to).