My wife and I are considering buying a plot of land and either building a custom build on it or putting a manufactured or modular home on it.

A new/improved road here will also benefit them as well if you are bringing more customers to their shopping center and providing more access points for traffic flow in/out. 

While this option may require permits for the renovation work, it may be a simpler process compared to replacing the entire home.Ultimately, the decision to replace or renovate the mobile home will depend on your budget, timeline, desired level of customization, and future resale value.

Customer service is unwilling to refund anything on, essentially, a defective product. 

My main source of income is a full time W-2 job with a custom home building company (S-Corp).

And if you become a repeat customer they may do the same for you. 

If your property is located in the city of Detroit you can transfer the water to the tenant all you need to do is setup your account as a landlord account and you need to deposit $150, then once a new tenant comes in you tell the tenant to sign up as a new customer with the city and the water will be billed to their name and SSN. 

It's a big project (we went the custom route) but I'll operate it as a STR and expect it will gross $4,500/mo or more.While appraisals may not have caught up yet, I think you'll see strong sales prices for properties with ADU's.

So you need to determine who your customer is, what is your value proposition and how you get them over the finish line whether it be online in person or a mix of both

We spent 10 years and then sailed away on a sailboat paid for with tax free dollars, 3 waterfront vacation rentals paying for the sailing, and a custom built dock in the back of one for when we were in town.