Being the former king of doing things on the cheap, I can offer some advice. My first suggestion is notice that I am the FORMER king...I learned that although many projects are tons of fun, and I would get great satisfaction out of building stuff, I learned that people who build stuff professionally can usually buy better parts, more cheaply than I can get them. Plus, they have the jigs and tools to put these parts together better than I can.
I still enjoy doing some of those things. But I limit myself to things I KNOW I can do. Recently, I made and installed a tool steel blade into a folding knife that I had worn out. The knife had sentimental value, and although it took me nearly 30 hours to fabricate, heat treat, and install the blade, time and cost weren't important.
I really suggest you follow the advice that precedes mine, and either buy a tumbler, or buy a motor that is ready to work. Just fabricating a stable mounting system for your motor will likely be a major effort. A used rock tumbler can't be all that expensive.