Not all repos are necessary, but they couldnt hurt. that is unless they are offline/unavailable and then you start getting errors in the log about that. some apps may have dependencies for apps in other repos, so its best to have a complete set of repos, just in case.

a full battery charge takes between 4 and 6 hours i think.

many of the apps do have overlapping functionality, but i dont think their feature sets are a 1 to 1 mapping. mplayer can play more media types than most other players available for the nokia. canola is more feature rich than media streamer, and has more eye candy.

too many downloads, i dont know what that number is but it occurs when the onboard memory fills. about 64 MB on the 770 i think. you can install a ton of stuff though.

good luck, welcome and happy hacking