I got lucky but they're not too far out of reach. There's multiple upgradable MacMini models out there, but they newest models aren't user upgradable. First check out this page at EveryMac. You're looking for models inbetween the release of Mini's with no optical drive and current. https://everymac.com/systems/apple/mac_mini/index-...
The first system is Mid 2011 Corei5. That's what I bought.https://everymac.com/systems/apple/mac_mini/specs/...
Late 2012 is the last fully user upgradable model. They had no 2013 releases. The 2014's have soldered ram and after 2014 they didn't release any new models since in the Mini line. There's rumors of a 2018 Mini but they may also scrap the entire line alltogether.
These are really easy to upgrade. Memory is SODIMM models like a laptop. Processor I haven't tried, but hard drives are easy too. You need a motherboard removal tool in order to slide the motherboard (logic board in mac lingo) out about in inch to install a second hard drive easier. It is a two pronged loop you pop into motherboard holes to pull it out. They're cheap. You will need a drive ribbon accessory to install a second drive.
I bought my parts from iFixit which also has tutorials for each of the models. https://www.ifixit.com/Device/Mac_Mini_Unibody
They sell the toolkit, motherboard tool and second drive ribbon for $27, but I've seen it cheaper on eBay and Amazon. All these Mini's have a bottom that unscrews to reveal the internals. If you need a copy of Mac OS Sierra/High Sierra, I can send you one. I make these regularly at work on USB flash drives. In my experience High Sierra runs best on this model with 8GB ram and a SSD. I have tried regular Sierra on a hard drive and I didn't like the speed feel. It works but the SSD really helps.
In my case I have it connected via HDMI to my TV and I'm making it into an emulation station.