I am building a 2 unit home in Hill City, SD.  It will have a 2 bedroom 1 bath apartment above a 2 car garage.  672 square feet.  This will be used as a vacation home for me and my wife.  On average we will be there 3 or 4 days a month. This unit will have cove heaters as backup heat.  When we are not there it will have the heat set at 50 in the winter and a/c at 80 in the summer. 

There will be a 720 square foot slab on grade addition behind and around one side of the garage. (L shaped). This will be a vacation rental.   2 bedrooms, 1 and 1/2 baths plus sleeping loft.  Average ceiling height 10 feet.  This unit will have an electric boiler in floor heat to keep the slab warm in winter.  2 inch insulation under the slab.  

I’m thinking of using Mitsubishi hyper heat heat pumps for each unit.  Should they be 2 separate systems?  Or all on one condenser and just zoned?  Will 1 head unit up and 2 down be sufficient?  Maybe some well placed ceiling fans can help move the air around?  Is there a better choice than Mitsubishi? 

What is your experience with heating in the cold and energy consumption? Any ideas on what size units?  Is going bigger better?  Finally, I am pretty handy, I have been an underground sprinkler contractor for 17 years and have built 11 homes.  I was planing on mounting the units myself and then paying a HVAC guy to test the lines.  Any experience DIYing this install?   

Thanks in advance for the advice. 

Dan