The 20gb ps3 should be fully backwards compatible. I think if you have 4 USB ports on the front and the flap on the left opens up, you are good to go with regards to ps2 games. The main disadvantage is the lack of WiFi on that model. Well that and the high failure rate unfortunately.
As for value I'm not sure as they seem to be a lot less common than the 60gb ones (in fact I don't think it was released in Europe?) .
Edit: the 20gb doesn't actually have the flap for memory cards or 4 USB ports. But if the model is CECHB it's backwards compatible Astrix2021-03-09 14:12:57