I've lived most of my life with septic systems, almost as long with a well (my childhood home had city water coming in but it went out into a septic system).
It's important to know exactly what your waste (septic) system is... there have been a lot of changes over the last 40 odd years, with more modern systems being more "green," more sustainable, and more expensive. Our present system (built 1978) is a traditional tank and drainfield. The problem is, even if you get your tank pumped regularly (every few years, depending on how much water your family uses), sooner or later the drainfield gets silted up and you need to have a new field dug. And because we are on a lake and there are restrictions about how close to it you can have a drainfield, AND we have a well and you can't put your drainfield too close to the well

scared1: ), we don't have a good place to site the new drainfield. So we will be going to a better but more expensive system, requiring more maintenance, when the drainfield goes.
So you need to know:
Exactly where (with maps of the lot, showing how deep as well as where on the lot) is the well, the tanks, the pipes, the drainfield? Get a complete map of the system.
How old is every component of the system?
What is the current building code in your area for septic systems... it may have changed since yours was built, and that means any replacement system will need to meet current code.
What kinds of maintenance have the current owners been doing (I personally wouldn't buy if nobody's opened the tank up to check it within the last year or two, usually when it's pumped out by the honeydippers!)
(Well) How deep is the well? How deep are the neighbors' wells? Ever run out of water? Neighbors ever run out of water?
Also, for the well water, I'd ask to see recent water quality tests, looking for bacteria AND checking the level of any undesirable minerals common in the area. (In our area, that would be uranium.) Usually in our area, part of the house inspection before sale includes running a sample of tap water and getting a screening... but it would be nice to have some info before that point.