Even if you use some of the cardboard rolls for covering flooring during paint/demo you can't go wrong.

Unless of course, you are looking in Boston - then all bets are off.Your offer for the property should be land value minus demo costs. 

They also left a bunch of garbage from the demo / construction in the yard.

Here are photos of a pool demo at my rental... $3,000 labor, material and equipment, one days work.... in Visalia California.....

We are actually doing a live demo on Thursday at 5pm PST.

How much is the demo cost?

Most of the harris county properties at the price point (and a bit higher) its highly likely that you will end up getting an empty lot with a run down building which may or may be there by the time you make acquisition (i.e. city already sent notice and may demo it for you) ...i would take that 30K and find a rehab house and use hard money to buy and your funds to repair and refi to take all cash out and start renting .. 

The first one they want someone with plans to demo house and build a new house on site.  

That’s what I saw you had to do from their demo video.

Without knowing your area or market this is where i would start.Maybe the lot had a house that was demoed.