Hey Steve, thought I would throw my two cents in as well! As you know I just started this wonderful/terrible hobby! I bought seven molds to start with:
Starship Wall Mold #301
Cargo Bay Accessory Mold #302
Med Lab Accessory Mold #303
Starship Deck Mold #270
Laboratory Floor Mold #271
Service Area Floor Mold #272
Pipeline Floor Mold #273
A little expensive to start with, in reality you should only need two or three, but I wanted the whole experience. Aside from using bad plaster, the only thing I can get locally, I have had not many problems with these. A few molds are really prone to air bubbles, just because of the fine detail, but most are easy to use. I now use a tooth pick to get into the molds and help release the air bubbles. I'm not sure sure how the castle molds are but some of these are tricky. Other than that I don't think that Sci-fi molds or castle type molds are not that much different.
2. Walls and floors are a must, then either the cargo bay, or med lab. The Service area and Pipe molds are not really needed.
3. Check out the wizards miniatures on ebay or where you can find them cheaply. Battle droids and clonetroopers are a must then for your boys. Also see if you can get some Jedi, they should really like them.
Well, I have not said much that no one else has, but hope this helps.
R2builder