An original Dualshock (PS1) controller is probably easier than a DS2 since most of the buttons are not analog. IIRC, the shoulder buttons are hall effect, which causes problems for typical "controller hacks". There are also some "purely digital" controllers out there. They tend to be the cheap third party ones with no analog sticks. These are generally the most ideal "controller hack" candidates, and they tend to be really cheap, too, as a bonus.
It's probably easier to get an encoder that speaks the same language as a Dualshock to the PS2, if you can find such a thing. I know there were at some point a few available, but with the waning popularity of the system, they may be gone. Somebody probably has a project to turn an Arduino dev board or similar into one. The protocol is pretty straightforward (much simpler than USB, actually).