Well just to get it working, you would need soldering equipment and a battery. To actually reprogram it, you would need a FTDI breakout, which is a great tool in general even if you are not working with arduino. I use mine in developing all of my projects.
Arduino’s IDE is free to download. Again the only other investment would be the FTDI chip. Which I love so even if you don’t go through with doing your project, get because its such a nice cheap debugging tool.