It sounds like you've really thought through not just what a good deal looks like today, but the other factors that will contribute to a more long-term appreciating value.

So, if the seller is unable to do it from their proceeds or something, then I would budget for that amount out of pocket if it is still a good deal at those terms? 

I live in the Portland, Maine area and am currently launching my real estate wholesaling business as a short term strategy in order to build knowledge, capital, and a network so that I can eventually start investing in my own projects such as fix and flips, and both short and long term rentals.

If the unit they are currently renting does not meet that requirement, then they will need to move to a unit that does.The landlord is under no obligation to lower their rent or change their terms for an existing tenant.  

Longer terms are available as this is considered a second home.

Can someone help me with terms? 

I co-own a short-term property management company.

However, I will say, if it is a year payment, it is advisable to hold the year's rent in trust and pull the money out as each month's rent accrues, just in case there is a dispute during the term.

With your proposed seller finance terms, you'd see serious negative cash flow for years! 

The long term play would be to pivot to MTR/LTF afterwards.