Wow, that's an interesting situation. Information is definitely your
friend here. The first question is, does the house indeed have septic
or is it hooked up to a city sewer. I would not assume anything. If
it does have septic, you need to know where is the tank, and where is
the leach field. An easement, if properly drawn up and recorded, can
give you the right to continue this arrangement, but it could be kinda
awkward. You might want to have evaluations done by (a) lawyer and (b)
plumber. Write back to tell us who charges more!

One note of advice: if putting in a new septic system, it might be
better to have them knock the cost off the selling price and have it
put in yourself, if you can make the financials work. That way the
work is done to your liking and specifications, not theirs. You have
the greatest incentive to ensure it is done well. Speaking of which,
is there a well also? -- H