Help me with my geriatirc iMac

[ka jowct]

I think your machine has USB2, so a FW option might be better for an external SSD. OWC sells some small enclosures that have the option to run them with an AC adapter, which is probably preferable if the new drive is going to be the main boot drive. One is aluminum (Elite Pro Mini, I think) and one is transparent. I have both enclosures. Both should also have USB 3 so that you can also use them in the future on USB 3 with a more recent system.

I have SSDs from Crucial and Sandisk in use. No complaints.