If you have an old MTB with a steel quill stem it's no problem at all. Get a Ritchey roadbar wit a 25.8 diameter and it will fit into a 25.4 stem with ease. Or get a japanese roadbar thathas a 25.4 diameter.
Most oldstyle MTB-brakes will work with roadstyle handles although you will have to set them up pretty carefully. V-brakes is another matter.