Well, technically up-framing won't make it look bad, it just won't change the look. If you try to render a 24 FPS video as a 30 FPS video, it will still look like 24 FPS. That's why I say to render it at the lowest frame rate, just so it all looks the same. You can get software that can interpolate additional frames, but it sounds like it would be unnecessary for what you are doing, and it would be extra money that you probably don't need to spend for this project. Plus, the results would vary. I'm referring to software such as Twixtor.
