Is there any one Canon P&S that would be considered a "reference platform" for CHDK which supports most or all of the CHDK feature set.
No, CHDK features depend on
1) the hardware available in that particular camera. (e.g. aperture control requires iris hardware)
2) someone has taken the time to implement various CHDK features that require extra code (e.g. extra long exposures)
3) some features require things that only exist in certain camera firmwares, or we don't know how to implement them on some cameras (e.g. DNG extension visible over usb)
Also if it has good features and picture quality. In other words, if I was going to buy a new or used camera that was the "best one" for CHDK, what would you recommend?
The one that is the best without CHDK (which you can probably get some idea of from review sites), unless that particular port has specific problems with CHDK features that you need.
The biggest influence on picture quality are sensor and optics. CHDK obviously can't affect either of these things.
I'm sorry to give such a useless answer, but the "best" camera depends on your specific requirements.
