SSD before Upgrading my Mac MIni

Buy the SSD of your choice and install it into an external USB3 enclosure.

Install either Yosemite or the El Capitan beta (I'd suggest Yosemite for now) onto the EXTERNAL SSD.

Leave your internal drive alone (for now).

Now, you have two boot options:
- internal for day-to-day work
- external for experimentation with the new OS

This way, if you don't care for the new OS, just disconnect it and stick with what you have been using.

DON'T "make the switch" until you KNOW that the new OS will do what you need and that you'll be satisfied with it.

 

If your only reason for upgrade is software it might be better either 1. Purchase used iWork 09 from eBay 2. Wait until most of the bugs in El Capitan have been fixed, probably around 10.11.3 or 10.11.4.

I recommend skipping Yosemite if you don't use iPhone or iPad, most of its improvements are related to iOS compatibility. While current 10.10.5 is stable there are still some bugs that haven't been fixed such as slow Finder, unreliable Software update, etc. Its possible you won't have issues but are Yosemites minor benefits worth updating?

If you wish to install Yosemite I recommend following Fishrrmans instructions.