I'd bet that most people here can't park their bikes indoors, so no, it's not a deal killer.

Some people use covers for their bikes when they have to park them outdoors. I don't need one since I can park under cover (though not indoors) if there is bad weather, so I can't really recommend any in particular.

I'd surely get a decent lock. Many recommend a redundant locking strategy.

Also, I'd surely get lights so you can be seen and so you can see where you are going.

Fenders is a pretty good idea for when it rains. I added mud flaps to mine and it has been a big improvement to the double rooster tails I used to feature.

Make sure you can handle a flat tire.

+1 on the merino wool and also keeping the shoes at work. I carry what I wear during the day in a waterproof bag -- socks and underwear don't take up much room or weigh much.

Depending on how you want to carry stuff, you might add a rack, or use a backpack or messenger bag. You can figure that out after you've been at it awhile.

Other than that, just ride.