Why not just buy something like the Motu 896HD, which has 8 built in mic preamps? They can be had used for $650-750 or so, which I would guess is roughly the cost of a used 828 and an 8 channel mixer.
Then you'd pretty much eliminate the need for the mixer, the pres in the 896 are better than the ones in the 828, and you *still* have the option to expand channels via ADAT later on if you need more preamps.
Yes it's only 8 preamps, but do you really need 7 mics for the drums?