I'm on a fixed income but I will try to switch to a SSD soon as I can. May take a while though.
Ideally, you would get the SSD size that covers all your storage needs, but this could be expensive.
If you wanted an improvement now, but can't cover the 500GB (or whatever size your HDD currently is), you can get a smaller SATA SSD, such as a 256GB which I have seen as low as $13, and a cheap enclosure or adapter cable for about $10.
Run the smaller SSD externally, keeping the OS and your apps on the SSD, and any files, photos, documents, etc. on the internal HDD. This would be a be improvement, but for less than $25.
After awhile, even though the internal hard drive wasn't used for anything, it started throwing lots of errors and causing the system to crash.
I have a lot of Macs, many of which are from the 2011-2013 years with HDDs or Fusion Drives, and I have also see failing HDDs cause really weird issues.
One of the issues somewhat recently was with Late 2013 iMacs, I have two of them with identical maxed out BTO specs, with the exception that one had a Fusion Drive and the other with just a HDD. My daughter used both of them, one at my house, and the other at her mothers'.
SSDs externally on both as boot drives, as the HDDs in both iMacs had totally failed, and you wouldn't even know the HDDs were there, with the exception of that sometimes when booting, there would be the "Initialize Disk" message that would pop up on the screen. If I would click on the "Initialize" and go into Disk First Aid, the HDDs wouldn't show up.
The only other issue is that it seemed like the "Chime" would take longer than it should when first starting, and the boot times were slower than expected with a SSD.
Other than that, both iMacs ran flawlessly on High Sierra and Mojave.
Earlier this year, I upgraded them both to Catalina, and the one with the Fusion Drive (again, failed HDD) ran flawlessly with Catalina. The one with just the failed HDD ran great for about 20 minutes, then would have a Kernel Panic, automatically restart, just to have the process repeat over and over again. Boot back into Mojave, no issues as all, it was just Catalina.
After a lot of troubleshooting, I decided to just swap the internal HDD for the external SATA SSD that I was using for it, and the Kernal Panic went away.
Not only did the Kernel Panic went away, but the chime behaved like normal, and the boot times dramatically improved. Once booted, there really wasn't a noticeable difference from using the SSD externally. Just ran well.
The point is, failing internal HDDs, even when not being used, can sometimes lead to unpredictable behavior. This wasn't the only example of have, I have many, but this was the most recent, and is on topic of the thread of upgrading to Catalina.
