You also can't determine what rent rates will be in 2-3 years so it's risky to lock in a rate and potentially be way below market.

It's just not going to function quite as an MHP would but still having knowledge in that space will allow you to determine if this would be profitable.Not buying just because you're afraid of having park owned homes is a bad idea.

You might run into adequate HVAC capacity and needing to run additional ductwork (then determining a pro-rata billing set up perhaps) or even an additional unit with separate water meter just the same. 

We have been wanting to invest in real estate for years and are determined to push through any obstacles but want to be smart about it!

A red flag is something that needs to be investigated to determine if there is mortgage fraud or not, it happens all the time. 

How I can determine if I am way overpaying for this property??

I recommend you do some research and learn your local laws so you're not misled by internet experts like myself. :) 

The rents used in determining self-sufficiency can actually include the unit you are going to occupy as well.

I'm trying to determine the best way to structure this real estate deal with my father-in-law(FIL). 

Unless you want to be part of a lawsuit to determine the legalities of this you may prefer to not get involved.