The GnM would be the worst possible device you could buy. It plays music, and I think movies, but its homebrew support is horrible. That said, let's get to the questions.
1. Pretty much any device will work for music and movies aside from the GnM, using the Moonshell application. You have to convert videos to the DPG file format first however.
2. The homebrew web browsers aren't that much to write home about. The best one is the one in DSOrganize, but even that is text-only. The best one is the Opera browser.
For slot-1 devices, the recommended ones are the R4, M3 Simply, and Supercard DS. You can tack on the $20 EZ-V 3in1 expansion pack if you want to use GBA homebrew as well. It's currently mostly available in Lite-sized form, though a few retailers have started selling Phat-sized versions.
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