@Benjamin Schultz I have no knowledge of tax statues or procedures in Maine/New England. Obviously every municipality has their own way of doing things. But in speaking from experience and just in general, the condos have separate tax parcels assigned to each so it will be taxed differently than an apartment building. I see this all the time in NYC when you compare taxes on condos and coops to apartment rental buildings. I'd call the assessor's office of the local municipality to see if you have any options to apply for a re-assessment after the sale. I'd imagine they have a set of procedures in place. I would also run your numbers with the current taxes and consider any possible tax adjustment gravy after the fact since it's not guaranteed to be reduced.
I also wouldn't worry about owning only 18 of 19 units. You control the condo board so the only issue you will run into is if #19 stops paying dues which wouldn't adversely affect your bottom line. A number of lenders would finance this transaction.
Hope this helps.