I want to install a SSD. Which one and how

First your current drive most likely needs a little TLC to get its performance back. I would do that and make sure its in good health before adding the SSD.

• Start things off by getting an App like this one: Disk Doctor so you can clean out the cache & log files.
• Next create a bootable USB thumb drive (16 GB or larger) using it, boot up your system so you can run Disk Utility to check your drive repair permissions & disk.
• I would then run a HD defrag app like this one: Drive Genius

Once done your drive should now have the zip it had when new!

Lets first upgrade your memory and make sure it's working. As this is real easy to do in this system.

OK, lets move on to the SSD side of your question: This system can support two SATA III (6.0 Gb/s) drives. While you could get an adapter to use either a M.2 or mSATA SSD it won't offer any improvement is performance and will cost more between the adapter and the more expensive SSD. Bottom line here you have the needed SSD in your hands now your Samsung 2.5" SSD. But! you do need the SATA cable, screws and the needed tool to open your system to add it in. Here is what you need: Mac Mini Dual Drive Kit.

Here is the IFIXIT guide you'll need to follow: Installing Mac Mini Late 2012 Dual Hard Drive Kit. Once you have installed the SSD you'll need to prep it using Disk Utility to create the GUID partition mapping and create a Mac OS Extended (Journaled) partition. The last step here is to install the OS using the OS installer.

Now lets talk about your question on El Capitan. Should you install it or stay with Mavericks. Generally I recommend upgrading to the latest as it often has security fixes that just can't be added in the older OS releases.

El Capitan is just fine but it does have some quirks. Disk Utility got a make over which makes it easier for a novice but does pull out some things we use. So I recommend creating a bootable USB thumb drive with Mavericks so you have the better Disk Utility. Besides I often need it on its own drive any ways so this is not a show stopper just being proactive. Bottom line I would go with El Capitan we have it on over 300 systems now and will get it installed over the next weeks to all of our systems.

So lets do a quick run down:

• Need to do some house cleaning on your HD and make sure its OK
• Need to use 2.5" SSD and need 2nd drive hardware Vs swap out to get the most bang here. Will need to install the OS on it.
• I would still have the HD in the system, just alter things so the SSD is the boot drive and has the Apps on it leaving the HD for my deep storage of my docs, songs, movies etc...
• Moving to El Capitan is just fine. It does have a few quirks which I think will be fixed up over the next update or two.
• Adding RAM is always a good idea.