You didn't indicate how much RAM which processor you have. You'll want at least 16 GB of RAM on board; everything just wants more memory now. As for replacing the HDD with a SSD though, I'm not sure why others seem to hesitate to recommend this. For my money, there is not a single upgrade I've made to a computer that has benefitted me more than an SSD upgrade. I've done it on perhaps a dozen computers by now, some Macs, some PCs, mostly for users other than myself, and everyone has been extremely pleased with the result.
One of the Macs I did was my office iMac which is also a 2012, but 21.5". The hard drive was failing and, as it was a business machine and already 5 years old, it was a candidate for replacement. I thought it was worth trying the SSD upgrade though. When the work was completed, there was an almost alarming difference in overall system performance. I was extremely happy with the results. (Opening up the case is not for the faint of heart, however, and you'll want to have your ducks in a row in terms of the parts kit to reassemble properly.)
Final note on SSD capacity; I've gotten in the habit of moving media files to external storage. This allows me to generally use 256 GB SSDs, although sometimes I've opted for 512. The solution works well.
Anyway, you know what kind of work you do. If you feel you are processor-bound, then none of these upgrades is really going to help much. While more RAM and faster storage are always nice, you may simply need a faster processor, in which case the refurbs may be just the ticket. On the other hand, your existing iMac is going to be a good starting point for ~someone~ to upgrade, and that person may be you.
