Is it advisable to plug the iMac into a surge protector?

I also recommend products from both APC and Tripp-Lite. Knowing how no-name manufacturers like to cut corners to ridiculous extremes, saving literally a few cents here and there, I would never trust my products to them.

A UPS adds battery backup to surge protection, and is ideal.

Yes it is. I have several of the following surge protectors, that I use for my Mac desktops and entertainment center.

https://www.officedepot.com/a/products/9828233/APC-Home-Office-SurgeArrest-8-Outlet/?

Both APC and Tripp have been in the power management business a long time (APC: 1981; Tripp: 1922).

I only use their battery backups. Much safer than just a surge protector, and the prices are getting more attractive compared to 15 years ago. Tripp also makes surge protectors that I trust.

Add up the wattage for all your gear including your backup devices and such (and possibly also for a LED desk light) and choose your ride-over run-time, and this will size the battery-backup supply for you. A desk light? Gets very dark trying to shut down, when the power goes out at night.

Most battery-backup vendors will have sizing apps available on their websites.

I primarily use APC battery backup (APC Back-UPS or Smart-UPS, depending in context), though Tripp Lite also works well.

For some installs and cabling, acquiring one-foot electrical extension cords can be very helpful too, as that allows moving the wall-warts away from what can be space-constrained array of sockets.

If you cannot afford a battery backup, then absolutely yes, a surge protector is advisable. Again, choose one properly sized for the wattage of your gear.

This is what I use:

It never hurts but using a UPS is a much wiser choice especially if you live in an area that occasionally loses power.