Does anybody know how to customize Project M with only one or two programs? I don't want to have to install very many programs.
It really depends on what you want to customize in PM, but for the most basic things, you would be just fine using brawlbox. If you are looking to use content that others have made, and maybe throw in a few tweaks of your own, I would recommend installing brawlbox, and GIMP.
I have compiled some useful tools and websites for you here:
http://forums.kc-mm.com/Gallery/BrawlView.php (Brawl Vault: Textures, Stages, Audio, Tools and resources, etc.)
http://modulous.net/ (Modulous: Another website hosting Brawl / PM mods)
http://www.lakora.us/rel/ (Brawl Stage Module Converter: A useful tool that allows you to replace almost any stage with a custom stage)
http://www.lakora.us/brawl/ (Brawl Utilities: Useful tools for adding custom stages, costumes, music, etc.)
http://www.brawlcustommusic.com/ (All sorts of custom music and other audio; typically "BRSTMs")
I think this should get you started. If you have any other questions, I will try to the best of my knowledge to answer them. There are also tons of helpful people on the smashboards that know a lot more than I do. Best of luck to you, and have fun!