Two questions I hope someone can help me with...

Just bought a three family home-2nd and 3rd floor apartments appear to have had vent-free wall mount units at one time (have been vacant for awhile)-Are these units effective, safe, and effecient means of heating these days or should I be considering something else altogether?

I will be looking for a HVAC professional-what measures should I use, questions should I ask, etc. to make sure I find a competent professional company/individual?