Jordan,
There is no fast, easy, or cheap way to do the type of renderings you want. I have used CAD before to do a few renderings but it took a while and didn’t turn out that great, and I am pretty experienced with it. I know of a program that offers a free educational license, but I wouldn’t recommend trying to learn it for this type of a project, it will be far too time consuming.
As Ryan suggested you can use Sketchup, which is free and is the easiest 3D rendering program to learn, however it probably won’t turn out the way you are hoping. The graphics aren’t very professional looking, unless you want to spend the time enhancing them in Photoshop, but that could take a while. If you do decide to use it, it is fairly easy to import a scanned image of a topo map and explode the contours to give it a 3D effect, however anyway you slice it there will be a learning curve and to achieve decent results will be very time consuming.
Lets just say that as an aspiring architect I hope to never have to provide my clients with computer rendered 3D models. Because good design is always finalized in the field, I believe it to be a tremendous waste of time and money to create renderings in order to help your clients understand your ideas, I would much prefer to present them with hand drawn sketches. Have you considered such a possibility for your project? I would recommend exploring an alternative to computer renderings, it will save you from a lot of frustration and wasted time.