For MWO, CPU is important. Going from dual core to quad core makes way more difference than any graphics card will. On the other hand, going from 4th gen to 7th gen didn't do anything noticeable in (MWO) performance for me.
Runs well enough on 4th gen i5 (Haswell), roughly 60-70% load on all cores. Would't even consider i7 for budget gaming, unless you happen to get one for the same gen i5 price.
Be aware that some office desktops come with non standard power suppliers and motherboards, meaning that you might not get a graphics card to work well even though it fits in. Have had a ton of problems with that in my Dell Optiplex. It just boots with black screen most of the time with any newer GeForce (700 series and forward) and last bios update is from 2014.
I'd just go for used 4th gen gaming rig and be done with it. Shouldn't be too expensive. GTX 770 is more than enough for MWO and many other games and at least where I live, they go under $100 (as it's not suitable for cryptomining.) Needs a roughly 500W PSU though. If that is not an option, then GTX1050ti or GTX1060 would be reasonable options. (I have 1060 running on 365W power without issues, total peak consumption metered for the whole rig is just under 200W when gaming.)
Edited by xRatas, 12 March 2018 - 11:00 PM.