You dont need that good of a computer to run sc2. But if you are to run it on really high setting then you will need a good cpu instead of gpu as the game is pretty cpu dependent. I built mine for under $800 on newegg and it serves me very well.
Make sure to get an SSD and install windows on it, much easier when you buy the computer instead of owning it for a month and then buying because then having to re-install windows.
Even the smallest donations help keep sc2sea running! All donations go towards helping our site run including our monthly server hosting fees and sc2sea sponsored community tournaments we host. Find out more here.