Most expensive part will be STI levers. If you already have a 31.8 stem, that will help, as road bars come in that clamp diameter too. If you've a 25.4 stem, you'll need a new one of those, too. Although I think there are one or two bars with 25.4 clamp and proper diameter for road brifters.

In your last sentence you say you want road tires, too??? It would be easier and better to just get a low end road bike, or even better - a nice used one. Oh, and check out cyclocross bikes too.