Put in a new HW tank as the old one was only 30 gallons & they said they ran out of hot water.

As far as the water temperature have you checked the settings on the tank ? A

I was once replacing a toilet in a unit because the tank lid cracked and we couldn't find one that fit the tank. A

The sellers bought it as a foreclosure but were aware that the septic had to be changed for other reasons (metal tank as previously used for trailers) so they are agreeing to do a 30K escrow/ credit.

It has a active oil tank as well. 

The one ancillary I check & replace immediately is the expansion tank for the hot water tank as I often find them shot so I replace them to avoid issues.

A good team will make the best of a mediocre asset, and a bad team will tank a great asset.

The limitation is the ability of the soil to accept fluids in the leachfield downstream of the septic tank.  A

I doubt a seller will want to remove a working tank, as that would also require a conversion to natural gas and likely a new boiler.

I didn't read the entire thread but, If you decide to repair yourself, there are typically 2 heating elements per tank, a top and bottom element.