+1 on finding a good used bike. And, I commend you on wanting to start commuting - regardless of the basic reasoning, whether it is the price of gas or for health. A couple of points for you to consider:

It doesn't matter what style/type of bike you get, spend a few bucks at an LBS and have it fitted. You'll like the ride much better since it will be more comfortable.

Once you have the bike set up, start slowly. Don't try and kill yourself thinking you have to increase your mileage by 5 miles a week or something. You'll get discouraged and stop riding.

If you have pain - STOP riding until you get it checked out and find out why. Most pain issues can be 'cured' by adjustments to the bike or how you ride. Some can't - I now ride a recumbent because I have to much shoulder and neck pain from a DF bike.

If you can make it fun, you'll keep riding. Try some 'fun' rides on the weekend to keep it fresh. You won't have to be some place by a certain time. So if you're a little slow or tired, so what. Just a mile or two to keep you loose.

10 years ago I was commuting 20 miles a day and then 50-75 mile rides on Sat and Sun. Then I got a job where it was impossible to ride - an over-the-road truck driver. Now I'm back to commuting 3-4 days a week but, I had to start slowly again - that 7 mile one-way ride kicked my butt for quite a while.