1) there are emulators for all of those but Nintendo DS is extremely slow and thus not worth it. N64 is also slow but for that it really depends on the game. There's a good amount of N64 games that play quite well.
2) the ONLY way on the Wii U is to use a WiiMote + Classic Controller. If you have a adapter that plugs a GC controller to a WiiMotes CC port that should work too. You cannot use the GamePad and the only "emulator" that can use the Wii U ProCon (so far) is Devolution which USB loads your GameCube games.
3) Answer 1 pretty much explains this, emulating DS (on Wii/Wii U) is not worth the time. But, even if it did work perfectly, no. The Wii U mode is not hacked yet, only vWii so the Wii U GamePad cannot be used for Wii Homebrew. As mentioned in the previous answer, the Wii U ProCon is the only exception and can only be used with Devolution.
4) The Wii U mode is not hacked yet. There is absolutely no way at this moment to downgrade. The only option for changing your firmware is the official method built into the system settings. You cannot select which version to get, you'll always get the latest one.
Final) yes, if you buy the 8gb model and find it's space is insufficient you can supplement it with a SD card and/or a externally powered USB hard drive.