The Plaster of Paris should work fine. It's been a long time since I worked with plaster. From what I remember after the initials hard shell was complete I covered it with a skim coat of plaster. One important step I remember. Spritz water over the scenic towels before you begin. It will try to absorb all the water out of the plaster you mixed and cause it not to set right. Will have a powdery feel to it rather than a hard as a rock feel. Whenever you complete a section. Wet that area before preceding to the next batch.
I used a plaster called Special White. A layout built in my area featured in MR used it and it was available locally. Used about 8 bags. I think any plaster will work well in this application. I think a lot of times modelers get caught up in fancy names on the bags.
If you haven't begun yet. You may want to look at a thing called Ground Goop. It's a concoction you mix yourself. Spreads easily. You can pre color it. Won't work for rock formations and such. But seems great at hills and such. Lou Sassi who writes for MR and has written a few books came up with it. If you Google him I'm sure there's info available. I think it's main ingrediants are Sculptamold and Latex Paint. iBest part you can mix a large batch and use it as you need it. Just keep it in a sealed container.
If you go the plaster route. It goes a lot easier with a helper and 2 bowls. My daughter who was about 10 at the time thought it was fun. While your working out of one bowl. Your helper can be cleaning and prepping the next one.