Quite a few games will work fine, and in fact Workbench 1.3 will work on a Kickstart 1.2 based A500. I have an early A500 with KS1.2 and it came with Workbench 1.2 and 1.3 in the box when it was new!

However later games will probably not all work as 1.3 A500's were the standard for gaming. Some games released from 1987 onwards might therefore not work. However I don't know the percentage. You could always get hold of a Kickstart 1.3 rom and swap them over if you did get a 1.2 A500.

Another thing to make sure of is that you have a 512KB trapdoor expansion in the A500 you eventually get. These are very hard to find on their own, but every A500 I've ever obtains has so far had the trapdoor expansion fitted. Most later A500 games required Kickstart 1.3 and 1MB of ram.

If I were you I would keep looking until you find a good condition A500 with KS1.3 and 1MB ram. hey appear all the time. Or have you considered an A600. I had an A600 as my main Amiga for a couple of years and got pretty much every Amiga game to work on it. Older games run using a ReLoKick 1.3 floppy disk.