While I’m sure MH Parks have there place - the homes themselves are viewed more as depreciating assets by lenders - more like cars, and less like real estate.  

The attached 2 car garage was converted to living space by the original owner.

Or used cars.

But I never liked "work like crazy and drive a Flintstones car and reuse plastic bags until you have $2M, and then put the entire $2M into stocks and hope for the best."

If a bank is loaning on a personal property- car, motorcycle, rv, mobile home they keep the title until it is paid off. 

Do not charge up any credit cards you have, take out new ones and do not buy a car, or finance anything else new until after the closing. 

Regarding the other types of loans, based on your explanation the conventional loan seems to (on average) have a lower down payment but takes into account the normal loan factors whereas the debt service loan doesn't but with a slightly higher down payment, proof of income generation through the property, and never being able to live in it.

Is it normal to give them rent discount? 

A lot of the investors that I work with will get all of their properties Section 8 approved, which guarantees the rent at a higher rate than you could normally get for it. 

I this era of high taxes, insurance, and historically normal rates (notice I didn't say high rates as they are currently somewhat the historical norm), it's hard to do a higher loan-to-value deal and get the rents to cover the PITI + any HOA.