Great boat! I'm pretty sure the P18 and P19 castings are identical, as are the original P19 rudders. The upgrade for the P19 steering involved larger rudder blades, extended gudgeons, and extended arms from the castings to the crossbar. There are still plenty of people using the original setup. The only thing you would need to get started is a longer crossbar.
The MX uses the same mast, just a square top main and much larger jib.
The main difference I know is that P19s are much more delicate than P18s. It's easy to put holes in P19s. Depending on your P18, they also have more controls; 4-way jib, centerboards, mast rotation, serious downhaul... but it's worth learning.
I don't know the rest of the answers.
Good luck!