One thing I haven't seen mentioned (forgive me if I somehow missed it) is that if your wife replaces too many feedings with formula without pumping she could possibly put her supply at risk. Her body produces on demand so if the demand isn't there on a regular basis her body will decrease its production accordingly. Once in a while for a break shouldn't be a problem though.
As for types of formula, if you really want to go that route (rather than expressed breast milk) you may find you have to try different brands and types before you find one your baby will tolerate. My son never accepted the ready made types, only powdered and only Similac advance. I actually did EBM in bottles most of the time 'till he was past 6 months old.