The house had outdated carpet throughout and luckily there we nice hardwood floors underneath which we had sanded and stained for about $3000.

Anything sinking suggests unstable soil below the footer, or an improper (or non-existent) footer.

I'd guess the treads and risers are nice wood underneath those too.As far as the parquet laid on top of the original floor, resulting in that 1/4 inch difference in floor height, you could have a custom bullnose built to match the floor height if you decide to keep that parquet overlay.

Or, should I make a lower offer, and buy as-is with the only stipulation being that the oil tank didn’t contaminate the soil.

The soil is very sandy and at the top of a hill, so we must be very careful with the grass on the property, as to not overgraze it.

I'm a structural engineer and in this part of Texas we usually have slab on grade without basements because we typically have expansive soils.

The soil borings came back as 20,000 ppm.

Can result in enormously costly issues stemming from chemicals leaching into the soil and cost well into six figures or more to remediate.

Floors: Sweep, mop, and wipe down all floor surfaces, including underneath appliances.

I am always in and out at sea so for me I think having a few rental properties to generate cashflow is the best way for us to get our feet underneath us, at least until I retire and am able to put more time into this.