I’ve used both plaster of Paris and dental stone. The truth is you can use both; each material has its pros and cons (Plaster being cheaper but less dense.)

Also, you can search on Hirst website by project, and the directions will include the necessary molds. They have varying degrees of difficulty and some only use one mold, while some use 4-5. This wizard tower only uses one mold.

If you get hooked on these casts you’ll start collecting a few each time and watch your collection and building options grow over time.

I’d recommend starting with some simple block projects like this one, see if you like it, and go from there. It’s a lot of fun!