I'd suggest a 2014 Mini, the "top" model, with 16gb of RAM.
Whether you opt for the "factory" version (1tb fusion drive) or "build to order" (for an SSD), is up to you...
So, what you're suggesting is a 2014 2.8GHz i5 with 16GB of RAM and Fusion drive or SSD. That will set OP back $1199. If you mean the "top model" it is the i5 3.0GHz which will set OP back $1399 for the same configuration.
My suggestion will cost OP about $750 for the Mac Mini (e.g.
http://www.ebay.com/itm/Apple-Mac-M...A-late-2012-/161815848690?hash=item25acf9e6f2). 16GB of compatible Crucial memory costs about $75. A 256GB Samsung 850 Pro SSD costs $140.
So:
2012 quad-core i7 with upgrades: $965
2014 dual-core i5 2.6GHz: $1199
2014 dual-core i5 3.0GHz: $1399
Oh, and then I forgot to mention the raw multi-core processing power of each, which matters in iMovie, FCPX etc:
2012 quad-core i7 2.3GHz: 11691
2014 dual-core i5 2.6GHz: 7038
2014 dual-core i5 3.0GHz: 7111
So, you're suggesting OP to pay 25-45% more for computers, that have a processing power that is about 40% lower than the 2012 model?