an SD Card alone can do everything for nintendont. that's how i set it up on Wii U to avoid all the HDD problems.
just format it properly to FAT32 and name your folders properly and it should work.
Class 10 works well. the Wii U'd reader doesn't benefit from faster cards anyway.
What sort of HDD problems exist? Is this just for Wii U? I have a 320GB drive hooked up to my Wii that I just recently put a bunch of GC games on. I've never had issues with the Wii games on there. Hoping it will be the same for GC games!