Handlebars are also a very personal thing. Unlike straight ATB bars, there are many different widths, bends, drops, and reaches for road bars. I size my bars by taking a measurement of my shouder width at the joint and using that width, but for average sized riders, a 42 or 44cm bar usually works. If you use clipless pedals, using the same ones on your road bike will mean you don't have to buy a new pair of shoes. Ultegra on my road bike and 747's on my mountain bike, and can use both my road and my mountain shoes with the Ultegra road SPD pedals.
Tires are pretty important, too.
I prefer a wider tire, 700x28c, because they are more resistant to flats, offer better rim protection, and a more comfortable ride. The limiting factor with wider tires is making sure they will fit in your frame, fork, or brake calipers. You can usually find end of season clearance deals on road bikes at your local shop or via mail order, so check them out. Another place to try is your local road racing club or team, racers will often sell old bikes and equipment at the end of the season so they can buy new the next year. If you have questions or any other tips for road bikes for mountain bikers, please email them to me.