Agree with above poster.
The older iMac probably has a slower (SATA-2?) internal bus.
A "premium" modern-day SSD will actually be "too fast" -- in other words, it will be "faster than the computer".
So... don't put extra $$$ towards the highest-end SSD, you'll be spending it for nothing.
I'd suggest an "economy model" SSD, speeds are not that important, because of the slow internal bus, the speed of the SSD won't make any difference.
All will deliver the same relative speeds.