<How do I calculate the finned tube requirement.>

By experiment.

Start by assuming a heat transfer coefficient for condensing steam of around 2kW/m².degC temperature difference, and go from there. Oh, and heat exchange calculations are nearly always based upon the log-mean temperature difference.

It is possible providing you have enough steam.

I'm not sure about your 35 MBH. Is that the heat needed just for the parts, or did you include losses?

What is your air flow? What is the air temperature going onto the fins? Can you recirculate the air? You are going to need to have the leaving air temperature as high as you can get it. That means the air will have to pass over several rows, like 4 or 6.

Are copper tubes & aluminum fins okay?

Contact www.srcoils.com and have someone size up a steam coil for you. Let me know what they come up with.