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I was fooling myself when I bought the lowest-end Mac Mini (late 2012). Without an SSD the poor thing was slow. The 4GB of RAM didn't help either.
RAM replacement was incredibly simple.
Adding an SSD was considerably trickier but the guide, and its comments, were very helpful.
In the end the whole thing fit back together perfectly and is working great. VASTLY faster, for the cost of a few hundred dollars and an afternoon.
The only real puzzle was the power supply, which was much more firmly stuck in place than I expected. It turns out that there are a couple of pins holding it in place, so you have to tug it more firmly than you might expect.
I elected to follow the advice in the comments and, instead of just adding the SSD as the second drive, I switched the manufacturer's drive to the secondary position and plugged the SSD in as a primary. This worked fine. After closing up the box I formatted the SSD using Disk Utility and then used SuperDuper to copy over the whole disk from the other internal disk, rebooted to the SSD, then erased the old hard disk.
Be very careful with the delicate little connectors. These things get smaller every year.
I've done a lot of electronics work in the past so I knew to move slowly and carefully, with good light.