Tips for getting enough bricks to make a MOC with unified style
Buying random LEGO sets hoping that it will have parts that will fit with what you want to build is an expensive process that can leave you with a huge number of unwanted pieces, and still not have what you want. 

Having said that, it is still a good idea to buy some sets that have a good number of pieces and/or building styles that you like and fit your project. For example, if you are planning to build a Star Wars scene, narrow down what scene you would like to build, and see if LEGO has already done any sets from that setting. 

LEGO actually considers all their sets as "seeds" that will hopefully inspire LEGO fans to expand on further. So, you buy that Star Wars set that fits your idea, then you start to expand on it. LEGO designers probably already worked out some of the most iconic features of that setting, so all you need to do is expand on them.

If you have a good number of parts, but they are in a rainbow of colors, you can still work on the project you want, but focus on the shapes and techniques rather than the colors. Once you are happy with the design, you can swap out the wrong colored pieces with the correct ones by purchasing them on BrickLink or at LEGO's Pick-A-Brick section.

It is sometimes difficult however to ignore a bunch of bright colors when all you want to build is a mostly gray space-station. So you can combine building with the bricks you have, and virtual building in LEGO Digital Designer. Once you are done, you can count exactly how many bricks you need to buy in which color.

In general I found that even if you are on a smaller budget or lacking space, it is a good idea to have a fairly large stack of bricks in one neutral color (grays, tans and white are the best in my experience), so you can model pretty much anything without being distracted by a rainbow of colors. This way you can work out the shape, size and design of pretty much anything, and then later switch out the pieces with the correct colors. This technique also significantly reduces the cost of projects, because you don't end up buying sets and pieces you don't want.

However if you are completely against shopping on BrickLink or at Pick-A-Brick, there are some sets that might work with your Star Wars project. I'm particularly thinking of the LEGO Minecraft sets because they contain large quantities of basic pieces in earth-tone colors (tans, browns, greens, grays, blues, white and black). They are pretty much the same as the LEGO Classic Brick Boxes that you purchased already, but with earthy tones. They are very useful for creating more realistic landscapes, buildings, etc.