SplitSecond S4

So its been only a month since i emptied my pockets, and now, just before Christmas she's making me spend again. "she" is a slightly modded 2000 S4. Welcome to life as an B5 S4 owner or what? lol

So the CEL came on the other day, scanned the ecu and came up with the following codes:
Bank 2 Sensor 1 (P1120)
Bank 1 Sensor 1 (P1113)
Some misfiring present (intermitten)

Im planning on replacing both the O2 sensors and all my spark plugs,what im wondering is this a common problem? MAybe i need a tune up? Can i expect this to happen again shortly? Im a relatively new S4 owner, forgive my ignorance. whats your take on the problem im experiencing? thanks for your time..

SplitSecond S4

..also im wondering how the audi dealership wants like 250$ for their O2 sensors wheras the prices online are relatively cheap.