Over the years Apple has made the mini less upgradable. And I agree the current version of Mac Mini is very limited!
Here's the IFIXIT teardowns so you can follow along:
* [guide|30410|Mac Mini Late 2014 Teardown]
* [guide|11262|Mac Mini Late 2012 Teardown]
* [guide|6131|Mac Mini Mid 2011 Teardown]
The 2014 model has soldered RAM you'll need to decide how much you'll need upfront as what you buy for the system will be what you'll be stuck with. It comes either with 8 GB or 16 GB of RAM.
The 2014 model also only offers a single SATA port {SATA III (6.0 Gb/s)} to support a standard 2'5" HD, SSHD or SSD drive. What would normally be the second drive bay is configured to support a custom Apple blade PCIe SSD drive. While the I/O of the custom Apple SSD is much faster than what the SATA port can offer you are limited to Apples SSD or one from [https://eshop.macsales.com/shop/ssd/owc/mac-mini|OWC SSD options when its offered]. The only rub here is you'll need to replace the Drive Tray to add in the Apple custom SSD if you don't buy a SSD configured system in addition to the SSD which is quite costly when bought as a spare. So I do recommend you think on what the largest SSD you'll want in this series and buy that model.
The 2012 & 2011 models offer two SATA bays. Which can support two or a combo of HD's, SSHD's or SSD's. Apple offered a Fusion Drive option in the 2012 model, one can create a Fusion Drive in the 2011 model as well. In either you must have a standard HD and a SSD drive, you can't use a SSHD or two SSD's. Here is the needed kit to add in the second drive: [product|IF171-005|Mac Mini Dual Drive Kit] and here is the IFIXIT guide to do it: [guide|6634|Installing Mac Mini Mid 2011 Dual Hard Drive Kit] or [guide|11713|Installing Mac Mini Late 2012 Dual Hard Drive Kit]
Both 2012 & 2011 offer support for two memory modules for a max of 16 GB of RAM.
So now you have the details on what is upgradable. Here's the specs for the different Mac Mini models to review: [http://www.everymac.com/systems/apple/mac_mini/index-macmini.html|EveryMac - Mac Mini listing]