Don't get an SSD if you're going for a cheap computer imo 1tb (~$90) > 128gb (~$90) SSD, but I have a ATI 5670 (~$50 now) and a phenom 2 965 (~$120) and I run the game on max with constant 60+ fps, you don't need a very good comp to play this game, if you are going for a cheap computer get ATI parts theyre cheaper and they aren't really that much worse than intel/nvidia 16gb of DDR3 ram is only like $110 now so you could easily get 8gb+ of ram for cheap
My prices on parts might be out of date haven't really looked at the prices recently, but you could easily build a computer only for tf2 for less than $600
For streaming though I've heard intel CPUs handle streaming lots better than AMD