Big one to check, is property tax up to date? I.E., make sure the owners didn't declare it uninhabitable to save a few quid.
The renovations may require planning permission, and touching more than 25% of the envelope will trigger the requirement to bring the property up to minimum B2 BER standard. What are the local authority permitting in that area?
In terms of the septic tank, depending on what's there and the age, replacing might require planning or might not be possible depending on the soil conditions and size of the site. Remember all work you do will have to be up to current standards, not what was acceptable at the time it was built.
I'd strongly advise engaging a professional working in the area to advise.