if you want productivity, you want an i7 or ryzen 7. more cores + threads help with rendering, multitasking and encoding. Some software companies utilize graphics cards for acceleration (such as vram); however, the processing is done by the processor. If you want a significant upgrade, you could try getting a i7-3770k
$250
https://www.amazon.com/gp/offer-listing/B007SZ0EOW/ref=dp_olp_all_mbc?ie=UTF8&condition=all
$200
http://www.ebay.com/itm/like/201812270805?lpid=82&chn=ps&ul_noapp=true
alternatively, you could get an AMD ryzen 7 or Intel i7 platform with new ddr4 memory, ssd and stuff. you'd probably have to get a new Windows license since the OS is tied to your motherboard. In my opinion, there's not really a good answer because it depends on your needs. If you feel like video editing takes too long to process, you can upgrade to a faster processor at your own expense. If you don't mind the wait times, go budget. Also used hardware is often a mix of budget and performance
People spend $270 on new gtx 1060's and rx 480's and sell them for $200. :O Or they buy new stuff and shave $10-50 on used. warranty is a joke - you spend countless hours undergoing RMA and shipping/handling. it's not worth paying for "new" electronics IMO. i'd rather search for stuff on craigslist/ebay/used amazon.
i3 is entry. i5 is mainstream. i7 is for productivity.