Most people I've seen convert MTBs to single speeds for the winter just remove the rear derailleur and then just take out enough links and choose the cog they want, since it's hard to find off the shelf 26" wheels with single speed axles that are also the right size to fit the MTB. Then leave the brakes on,

That's more of a "i have a crappy bike i don't want to spend money on doing a true conversion yet want less maintenance for the winter" solution.

Yes you want fenders definitely. For tires I like Schwalbe marathon winters but I ride only on always plowed streets where the main danger is black ice (though I've not seen it personally, it happens). If the streets might not be as good the Nokians are nice studded tires, with good tread as well as studs. If you go to the Peter White cycling site he reviews a ton of winter tires and shows how to pick which one for what conditions you'll be riding in.

Though a coworker got very cheap steel studded tires because his commute is only about a mile each way.

I notice the narrow tires cause me to get pushed side to side by ruts.

I have not really experienced black ice here, but there is ice in my alley for the last 10 feet of my commute and it's funny how i'm fine on the bike, and slip when I try to step off. So while i've not had problems, during the winter at almost every bike get together someone will say "oh i wish i had those tires when I fell last week!"