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My money would buy a 2012 Mini.
then a Samsung SSD with ifixit data double kit
16GB of RAM
802.11ac + BT4 chip

and call it a day.

2012 already has BT 4.0 and I'd actually call it 2 years at least.

Get a refurb if you can for $589, use refurb.com or refurbtracker.com to get updated when in the Apple store, as these go in about an hour or less.
 
If you will be install Samsung SSD in mac mini late 2012 (i7), there is some surprise.
You have to swap disks. SSD must be put in HDD place and connected to HDD original SATA cable. HDD going down and have to be connected to accessory cable.
Don't ask me why is that should be done. Only in THIS WAY osx see my SSD (Samsung <- maybe this is the key). I read about this also somewhere on the Internet. My two friends had also this same problem.

ifixit should update their guide

Yeah I've done this on my 2012 Mini, it was not that much of an issue and I gained a 1.25TB Fusion Drive in the process.
 
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