Do NOT update to the latest firmware first. If you do it, it'll wipe pretty much all of your homebrew (unless you have the latest version of the homebrew channel) and you will need to use a game exploit to rehack it. If you use modmii as it is and treat it as if it is an unhacked wii, you'll do the exact same without having to buy a game first.
Modmii does have a way to update the shopping channel, so that's okay. I also second xyzero's advice to get priiloader and bootmii (and a nand backup) first (details can, once again, be found within modmii). Oh, and update the HBC to 1.0.8 if it isn't already on that version.
CFG is a good loader, but if you use it to load your games from an USB drive, having cioscorp on your wii is at best useless (you're not using all those hacked IOS anyway...so why have them in the first place?). Darkcorp is the successor, but once again: only consider it if you want to use backup DVD's AND want to load those disks from the disc channel. For everything else, I'd say to just update the IOS'es manually (it's pretty much in the same modmii menu where you can download the latest shopping channel).