I've never tried 13 Gpu rigs but while a g39xx will probably be fine, i'd go with minimum 2 cores with 4 threads cpu to have more fun setting/using it. Combine with a ssd and it will be a charm to setup. Considering the price for 13 gpus it's not a big investment (+100$ for a i3 6100 and ssd).

Well that's just my 2 cents, obviously if you're short on budget then go with a g39xx and a used 2.5'' hdd.