This is common in the Chicago market.

Common mistakes can cost thousands.

Have you encountered any particularly common or unexpected challenges in your research so far?

Use them as guides in decision making not hard and fat rules.

@Alec Jacobs, If the electric dryer is on one units service the washing machine is as well.I'm assuming these are located in a commonly accessible location such as a shared basement.

I think you crossed 2 of the more common real estate tax benifits.Section 121: Primary Residence Capital Gains Tax Exclusion What it is: Excludes up to $250,000 ($500,000 if married) of capital gains when selling your primary home. 

. *** What loan structure and fees do you all commonly lock down and what would you recommend in my situation?

I won't dive into that evaluation here, since this thread is about due diligence, but just keep in mind every park is unique and without understanding all the variables, it's pretty common to misinterpret the value the park.

If so, what is a common percentage range?

Common fees will include a set-up fee, a leasing fee for each turnover or a lease renewal fee, marking up maintenance, retaining late fees, and more.