@Andrew Coulton You forgot to include lead paint, asbestos popcorn ceilings/walls and disintegrating iron cast sewer pipes, among other pleasures that come with structures built prior to 1980.​As for foundation, there is more to foundation repairs than just the foundation.

The dip tube inside the water heater was basically disintegrating and the tiny bits would clog up at the faucets.

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).

Im honestly at a loss as to these quotes. 4) See #3, but I honestly just say Im in the middle of an interior renovation, I dont have time to focus on the driveway (for example), Id like a quote please to have the disintegrated asphalt driveway replaced with concrete. 5) This is alluding to work thats started and unfinished. 

You could have foundation issues, or possibly old termite damage where major support beams have disintegrated or maybe support piers that have sunk or fallen over or rotted. 

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).

That means anything that would have been OK if it had been left in service - old knob and tube, for example, that wasn't disintegrated - will now have to be changed out.

That white stuff that looks like mold is actually called effervescence, which is the block sort of disintegrating from the moisture content.

Old houses have cast iron pipers that will disintegrate (because of age and/or foundation shift).

On that project I actually did have to replace part of the sewer too, the old iron pipes had disintegrated but that was the only exception to my staying out of the sewer rule.I've never heard of two inline water heaters but I don't see any reason why it wouldn't work if you have a spare one to use.