Construction cost is a major concern as you pointed out.

These lenders close faster, less docs/headaches [usually] and are where many investors prefer to go.To take it up a notch, if Cash-Flow is your primary concern and the name of the game, then there are also options for Interest-Only, through a 'Hybrid' term product.

Are there concerns with renting out the unit as a 3rd bonus unit?

That way, no concerns about out of province management.

If liabilities are concerns please talk to your attorney. 

However, my wife seems nervous a little bit to have a lawyer as a tenant, concerning their may abuse their professional advantage, victimizing the landlord and leading us to a possible vulnerable situation, as they may pick up on any tiny issue to make a big deal.

One concern is overpaying in this market and hurting cashflow which may be skimpy first 6-12 months. 

My concerns are that there might be some structural damage only in the sunroom.

The other concern would be if you pay utilities, this will increase water and electric bills.

@Daniel Walton, my concern with your numbers is you do not have any turn over costs, vacancy, repairs and maintenance, or Capex reserves accounted for.