Help me plan and acquire pieces for my Star Wars MOC? I wish to build a large LEGO Star Wars MOC.  The basic idea I had was a well-fortified Droid factory, protected by a combination of droids, Sith, bounty hunters and Manalorians.  And on the other side of this battle field: a Republic base built into the cliff side.

My issues are :


Planning a MOC of the size
getting hold of enough minifigures in a economic way
building large amounts of scenery cheaply


Any help will be appreciated greatly.
Here are some good pointers for building large scale projects:


Duplo is your friend. If you have duplo, or have the opportunity to buy duplo for cheap, it can be extremely useful. You can use it to easily build up the insides of landscape areas, without making everything obscenely dense. This will help with budget and weight. 
Build the base nice and strong. Make sure all of your baseplates are attached well to each other, and that you have a permanent building surface. My greatest recommendation would be to buy a sheet of plywood, and build on top of that. This way, if you ever need to move the model, you can pick it up in one piece. You can also make the model so that it splits in certain areas so that it is easy to move around. That is up to you, but make sure you are able to move things around and/or repair things easily.
For scenery that you need to replicate, allocate an exact number of parts for each of those pieces of scenery, and simply alter how you build it each time. It will speed up the process of building as well as add variety to the model.
Details come last. That being said, have an idea of what details you may want to incorporate as you are building some of the superstructures. This way, you can integrate things better in the future, and the model won't appear so haphazard.
Used pieces are an alternative method of coloration. Buying used pieces means that many of them will have slightly different tones of coloration as a result of age. When building in a large scale, this actually adds a really nice touch of realism to a model so that you have a much more diverse gradient of colors. 
Don't be afraid to use unorthodox building methods. When you're in a large scale, things can look very different since everything will be seen at once. When building, be sure to periodically take a step back from your work to see how a detail actually fits into the scene.


Hopefully this will help get your project off the ground!