On a 320k property Ill be paying 9k annually. 

Owners pay not to deal with problems.

@JR Gonzalez, You can allocate the proceeds from your sale anyway you want. 

Maintenance can also be a factor—while newer homes may require minimal upkeep, older units might need more frequent repairs, particularly for things like plumbing or HVAC systems.Some things to pay close attention to:Zoning and Permits: Check local zoning laws and permit requirements to ensure you’re allowed to place a mobile home on your land and rent it out.

We are building a place with HELOCs then after it is built we will finance it and pay off our HELOC, at least thats our plan. 

Be patient, it's going to pay off. 

Are there specific market indicators or property traits I should be paying attention to?

To get a GM, we need to pay a GM wage.

And in the same complex later built units had individual heat pumps and hot water heaters which the unit occupant pays. 

This is where it pays to consult with a real estate attorney.