Best if you have 25% going in so you have a little room for error.

Any sense of if this type of loan structure is widely common or specific to your locale?  

If he is suggesting a balloon payment with the seller financing, be sure that you could comfortably qualify for conventional financing, with a large margin of error if you needed to.

In your more northern clime though, gas heat is probably more efficient than electric heat pumps which are common around here.If the tenant really really wants an electric dryer, I'd offer to split the cost with them.

Look at comparable sales to see what kinds of features and finishes are common for properties in that price range. 

Little things that can add up to  be big things financially like garbage, water & common area electrical, landscaping.   

Population Growth in United StatesSeries : US Census ACS 5 - Population Estimate (2000-2018)Population growth is one of the most common and important measures when doing due diligence for Real Estate Investment.

Is this common with mortgage lenders or will other lenders potentially be more flexible?

As you know, it is very common for exchangers to complete their 1031 within the 45 day window. 

It's more common for their policies to discourage RE speculation rather than encourage it.