Then we setup these ozone machines and let them run for a few days all over the house.

Sometimes everything seems to happen at once (I had to repair a furnace, replace a dishwasher, washing machine and fridge in one year at one place-all appliances were original to the home and 3 decided to quit at the same time). 

Purchase the machine and make it part of the rental?

It seems like a bunch of people are using Batchskiptracing and claim the data is good but I just saw that deal machine is now including contact data for free.

Additionally, what are the typical maintenance costs for these machines?

Not only that, most of the "GPs" had been working in Tech their whole careers and had only been into real estate the past couple of years.Don't get me wrong, we have a lot of very smart passive tech-industry LPs, but you want a vertically integrated machine of humans with deep real estate experience.

If you hire a big name brand builder you main have to pay more on overhead (project manager, superintendent, heavy equipment and machines) and their profit versus if you hire a small builder, the overhead could be almost nothing, and lastly, if you are a good investor/customer, you maybe able to get a better deal (a builder is willing to build the house for less profit).

So it seems to me that folks are probably happy to sit on the money making machines rather than sell.

This also allows the existing coin slots to be used so if the card reader or wireless has a problem you can still run. 

And before you even start building in the site prep there again, you are going to have to pay for big earth moving machines, that run on that $5+ a gallon diesel, and pay for a truck to move those machines to and from your job site, and if those machines are big enough you have to pay for the permits for them to go down the road.