I'd agree with going for an A500 to complement an A1200. You're already sorted for AGA and WHDLoad games with the 1200.
My advice would be to get a Kickstart 1.2 A500. A few programs don't run on Kickstart 1.3, plus you're guaranteed to get the original non-ECS Fat Agnus chip. It seems most A500s with Kickstart 1.3 came with the ECS Agnus.
One program incompatible with Kickstart 1.3 is the old Soundtracker IV by D.O.C. If I remember correctly, its keyboard-handling code jumps directly to an absolute address in the Kickstart ROM(!).
There were quite a few games and demos which didn't work properly with the ECS Agnus. For example Double Dragon II, Z-Out and the Digital Force demos Ode to Ramon and Ode to Ramon II. (Though all of those have WHDLoad installers, so you could actually run them that way on your 1200.)
A 1.2 A500 with extra 512KB in the trapdoor should have the best out-of-the-box compatibility with old demos. For very old demos which don't run with extra RAM, you can turn it off using one of those custom nofastmem bootblocks.