I'd also consider the following:

Why was the septic system placed on the neighbors property to begin
with? It sounds like there is no suitable location on the property
you are considering buying and I would want to know why. If there is
no suitable location on your own property or connection to city sewer
available, I'd be fairly reluctant to buy this property. When the
current system goes kaput, I'd want the option of getting away from
having another one put in the same location.

And if you do want to proceed, as others have suggested, I'd make sure
the easement is legally recorded and airtight, by consulting with a
knowledgable attorney. The sellers should have all the info on this
available too, as it's not rocket science that this will come up as a
major issue in a sale.