I would recommend(on the bases your gtx 750ti is one without a power connector) look out for something like a Dell Optiplex 7010, I got one with an I5 2400 for £20, similar HP machines around that time are good enough as well but If you want to add a good graphics card you will need a power adaptor(think I payed ~ £12) to put a better power supply in it because HP's proprietary power supply crap.
I've put an I7-2700k and a GTX 1050ti in one and I can play Quake Champions at 144fps 1080p at low settings.
These dells are the ones where they started to have USB 3.0 so it's a good way to spot the better ones.
