If you're looking at a house, especially an older one, with a septic system, get an independent inspection done on it. The inspection sellers need to do prior to selling only needs to certify it's working. You want to know the age of it, the condition of the drainfield, etc. You don't want to get stuck having to replace an aging/failing system as it can be very expensive.

I say this from the experience of owing a mid-50's rancher years ago that had the original 50 year old system when we bought it. We had an issue a few years later that almost caused us to have to replace the entire thing. It was shitty and trust me when I say you don't want to deal with it.