When I asked him when is best for him for a house inspection, he will be traveling (his family is still home), and suggested we do it after he comes back in 3-4 weeks.

And closing costs of $4,200 work out to 1.2%, which seems low for attorney's fees, appraisal, inspection fees, pre-paid homeowner's insurance, origination fees, pest inspection, pre-paid property tax, recording fees, survey fees, title fees, transfer taxes, etc.Not sure how old the property is, but CapEx + Maintenance of 14% feels optimistic to me. 

I'm guessing this is a conventional buy, so make sure you have and use your inspection contingency to check for it being an approved triplex, and make sure you have your financing contingency in case the appraisal comes in low.

I've had some plumbing issues and am looking at buying a cheap, $100 camera on Amazon (example: https://www.amazon.com/Industrial-Endoscope-Oiiwak-Inspection-Waterproof/dp/B081J3LN61/ref=mp_s_a_1_8?

It was subject to financing and inspection but I have removed those.

You may want to look into how much the inspections cost for that in your area.

I recently realized the city requires all single-family rentals to undergo an inspection process and $65 application each time the property changes tenants.  

So you see a cheap property you like the price on it.. what are the exact questions you ask the wholesaler or seller before you decide if it’s worth your time putting in an offer and moving to inspection?

From some research it seems like water sensors near water appliances, quarterly inspections, water proofing under sinks are some ways.

The only way I see you being denied is if the property does not pass the physical inspection, which simply requires a clean home with no safety or health risks.