I know that you need an SD Card to hack your wii, but do you permanently need it? Can you just borrow someones SD Card, hack it and then return it? Or does it permanently need to be in your wii. Also, can i go online/use online features if I use the Bannerbombs method on 4.2?
For the most part you don't permanently need an SD if you have a HDD. There are some older apps and emulators that don't have USB support and will only run from SD though. I would recommend having at least one 2GB or larger SD card for your Wii. It's also useful for storing your save games when your NAND gets filled up or launching WiiWare/VC games through the SD channel in addition to homebrew.
You can go online just fine with a softmodded Wii as long as you don't cheat. If you cheat online Nintendo will permanently ban you.