I can second much of the advice here, namely avoiding using a tire that is too narrow on a 26" rim. Going too skinny will make the ride much harsher and only slightly more efficient. Very slightly. I am a very happy Schwalbe Marathon Supreme rider and can highly recommend these tires. The ride quality is quite good and the flat resistance is among the best tires available.
For fenders, the Planet Bike Cascadia MTB, Hardcore MTB or Full MTB will all keep you dry and your bike clean, as they offer full coverage. Axiom makes fenders of slightly better quality than PB, and prices tend to be about the same.
Other recommended items would be blinking front and rear lights (if you ride on public roads). The Planet Bike Blaze 1w or 2w gives great front visibility to drivers (in flash mode), and the PB Superflash (new model) and PDW Radbot 1000 offer decent bang for the buck out back.
Mountain bikes are easily adapted to commuting duty.
Welcome to BF!