upgrade hard drive to SSD

Looks like you have three choices for SSDs:

• External USB 3.0 SSD.
• Internal SATA SSD
• Internal blade SSD ( https://eshop.macsales.com/shop/ssd/owc/imac-27-inch/2013-2019 ).

Adding an USB 3.0 SSD, to use as your startup disk would be far the easiest course of action – if your internal drive is basically OK hardware-wise, and you're just trying to speed things up.

If you go into System Report, and look at the S.M.A.R.T. status of the internal drive, that may give you some clue as to whether it is failing. "Verified" is good news (though not an absolute bill of health). "Failing" means the drive is, or soon will be, causing issues.

An internal blade SSD would give you the fastest speeds, but would require surgery. There were some iMacs where Apple left off the connectors for blade SSDs if the machines did not ship with blade SSDs or Fusion Drives. I do not recall offhand whether this is one of them.

What issues are you having? Before going to the expense of replacing a drive make sure it's not any 3rd party software that might be causing the problem.

Download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

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I am not familiar with that model but generally those around that generation are very difficult to work on. Not impossible but you might have to do something such as cut through the adhesive holding the display glass and then glue it on again afterward. In between might involve removing a dozen or more components that you have to be careful not to damage anything. You can likely find discussions about this online and I would read those.

Since that model has USB3 ports which are pretty fast, you might wish to consider using an external boot drive..

Go to the web site OWC. They sell parts and what fits where. Obviously it will cost more than just parts alone but it will give you some idea. I'm guessing it might still use a SATA drive and depending upon the size drive you want it might cost $100-$200 for parts.

Labor will depend upon what local places charge. I have always done my own computer stuff. I have a 2012 iMac and I know I thought about replacing the RAM and upgrading the drive but I figured it wasn't worth it for a computer that old and the headache in in 30 steps involved. I just use an external boot drive and use the internal, slower one for media storage.

About those "problems"...

It could be the software you are using. Until recently I was using Mavericks OS and I would have to restart my computer every other day or risk it abruptly shutting down. It was most likely an old browser I was using that was using up hard drive space and then not giving it back until I restarted. It might have said it was giving it back, and then the computer would try to use the space and find it not available.

I then upgraded to High Sierra. Now the issue appears to be that as uptime goes by, there's an increasingly long interval if I try to highlight a word in a text document and then drag it to a different location. The drag capability doesn't engage for about 4 seconds or longer, and takes longer as uptime increases. So I basically exchanged one issue for another.