Here is my advice:1.) the people you work with matter more than the market you choose.

Whichever you choose, define pickup day, container location, contamination rules, and fines for overflow.

Next step: get two bids each way that include service upgrade, per‑unit metering, and operating cost estimates, then choose the lower all‑in over 5–10 years.

Start by choosing a reputable custodian and clarifying your lane: lending from the IRA to unrelated borrowers vs owning assets inside the IRA.

When you combine the stable, predictable cash flow that MHP investing provides with the tax benefits, it's easy to see why higher net worth, sophisticated investors choose mobile home park investing over MF, self storage, parking or many other asset classes. 

…I heard that there is flat fee services but you have to be carefulwhich one to choose.

The % your choose will depend on your risk tolerance but I like ~20% buffer for holding costs if your timeline gets extended and ~20%+ for your rehab for the items that the home inspection or walking the property doesn't catch that inevitably pop up.Hope that helps!

However, I am stuck on choosing when to start.

My move: price the top three projects, rank by rent/Appraisal impact, then compare HELOC vs refi vs card using total cost and timeline; choose the cheapest capital that gets those upgrades done inside six months.

Many carriers/programs choose not to offer coverage or do so at elevated pricing.That said, we have quite a few great programs for Detroit properties.As always, I suggest working with a broker that has multiple options rather that searching out one particular company.