They'll essentially put a holey tube under the building, piped to the roof so the gas can pass through and escape without seeping into the building. 

Also we are not doing ALL the renovations immediately, some of them will be done over the years of ownership, such as the roof.  

I'm in California where you can buy a house for 200k, but it would have no roof and be on a lot less than 1 acre.

His company is Bullseye Real Estate We've used him mostly for small scale repairs, i.e. build a bathroom, repair the roofing, patch drywall, paint, kitchen install, masonry repair (front steps) etc.

It was so backed up it caused the dishwasher to leak all over the hardwood floor, so now I have to fix some of the flooring too. 

In addition, owner loaned us $100,000 at 6% simple interest for one year to make major repairs (installed heat pumps, replaced boiler (for heating water only), replaced some siding and major roof repairs.)We were very clear with each other what we each wanted out of the deal.

This later confirmed why the toilet was leaking and needed to be reset in the first place -- they had 3 kids so I'm pretty sure that had a lot to do with it.  

Other than that, a refrigerant leak, the fan motor burns out, just throw it out and get a new one.

Or I could dig up an irrigation leak, zero knowledge required.

You may pay 1.25X - 1.5X for a roof because you aren't repeat business whereas a GC uses the same roofer many times a year and they get a great discount.