If you want to get good paint, I recommend using games workshop's. It is expensive but it is some of the best hobby paint out there.
Also I would go to Home Depot and Michaels to get pipes, wood, clay, etc for the ship and alien landscape.
For the juggernaut and derlict, I recommend you get either the NECA space jockey or the sideshow. I also recommend using the NECA eggs as well. Also i would buy one of those life size fake skeleton models that teachers use in their class, and use the bones to construct either and alien hive or the egg chamber.
I recommend you use mainly NECA figures, they have the most character pieces for the project.
If you want to get shots of the juggernaut and the nostromo, I recommend you buy smaller model sets, then build and paint those.
What is great about the original Alien is that a lot of the props, costumes, and settings were made with cool and simplistic items such as pipes, egg strays, cones, etc.
Good luck