I suggest reading your purchase offer carefully. There should be a clause or clauses that covers your options. Speak with your agent and voice your concerns. (assuming you are working with a Buyer's Agent.
If it is already way below market, you may want to ask for half. Or, ask for it all and settle for half. The worst they can do is say no.
When you say drain field, are you referring to the septic system? If so, can you tap into local sewer for a cost? Septic systems can be extremely expensive to replace depending on the local codes in your area.
I think the bank will likely want to throw money at the situation. As I stated in previous posts, gather estimates and all documentation from the health dept. Meet with a few HVAC guys and make your request. If they wont do it, walk and dont look back. There will be others.
PS-It drives me crazy when the banks do not leave the electricity on! Stupidity!