Not sure if these points were covered, but don't spend $500 on your first commuter. You can get a great used bike for around $200 or less. Spend the rest of the money on accessories like fenders, clothing, bags, racks, or whatever you find you'll need to commute comfortably. After a while of commuting, you'll really figure out what the right bike is for you, and you'll be glad you didn't waste $500. Also, don't just hop on the bike and go 15 miles. Even if your lungs, heart, and legs are in condition to do that, your butt is not. You need to break your bottom in slowly. Start out with shorter rides and work your way up.

I only commute 6 miles each way, and I just started. I also only ride in fair weather, but I would suggest a couple of things based on how I have done so far.

First, allow plenty of time on the way to work so you can take it easy. Pushing the pace on the way home works for me, but I don't want to be too tired when I get to work, plus my office has no shower facilities.

Second, ride the bike for a while before you start commuting. I can ride 40 miles with a few breaks, and the 6 miles to work, I don't need a break. Much of my riding other than the commute is on a gravel rail trail, and I often do 6 miles without a break. So I was confident that I would be able to commute. The week before I did my first commute, I did a 16 mile ride, where the first (and last) 6 miles was my commute route (or at least all but the last few blocks on the busy city streets). I think I would have potentially felt the commute was harder if I hadn't built up some good base miles, and ridden it first as part of a pleasure ride.

If you can, consider driving part way to work for a while, and ride the rest. I drive the first 2 miles (to avoid a couple of hills, and narrow roads with fast cars that I am not quite ready for) and it works for me. I am basically on multi-use paths for all but the mile closest to work. When I feel better about it, I will ride from my house, and it will be 8 miles each way.

I would actually suggest that you not consider commuting on a daily basis until you can handle the round trip without a long break. The reason being that although you will be doing 15 miles twice a day, with a long rest in between, it takes a toll (at least for me) to do A COMMUTE several days in a row, at least until I build up to it.

As far as buying a bike, I would suggest test riding a few to see what you are comfortable with, and I would give slight preference to a more aggressive hybrid. The reason is that the less frontal area the better in cases where you need to battle a head wind. I ride a Trek 7.3 FX, and it is right for me. I am also fortunate that the few times I have commuted, the wind is either calm, or at my back in the morning, with a little head wind on the way home, but I like the riding position to help me get a little extra speed with the same level of effort.