Zodiac, here's what I would do:
1. Download BootMii and install it as a boot2
2. Use the NAND backup feature it has, it backups EVERYTHING about your Wii, including your save files and so forth.
3. Try the AR channel, if it bricks, you'll know in which case you'll then use BootMii to put your old NAND back on there. If it doesn't, then you're fine.
.WADs that are trustworthy or made well generally work. I haven't had a .wad brick my Wii but, I would suggest doing some research work on the guy who made the .wad. Chances are, you can probably just as easily find the application for the HBC instead and use that just to be safer. .wads are typically made from HBC applications (unless they are a game of course) so, look for the HBC application version of it first. If you don't find it, install the .wad AFTER you've backed up your NAND with BootMii.