Welcome to commuting! Its way better than jogging imo. =)

Helmet quality doesn't matter especially if its at a bike shop. You get something with better style lighter more vents etc, but the cheapest helmet (if you want to go that route) is going to be fine.

If you are serious about wanting to get back to riding a mountain bike again and you don't want 2 bikes get a mountain bike to commute on. Its already been mentioned but get some tires that are smoother and it will be plenty fast and 7 miles is a pretty short commute. In e prosess you can be getting used to riding that bike and getting it dialed in so when you go ride it on trails it will feel good.

You can get a great deal on Craigslist in almost any market, but you have to be able to work on the bike yourself and know what you are looking for. I don't think its very hard, but its not for everyone. It sounds like you have enough cash to buy a new bike if you want to. What is your budget low and high range?