Buying a house with septic a bad idea?

[Ian K Laszlo]

Gettings

Looking for a 2 bedroom 1 bath 2 car garage to live for the next 7 years and than convert to a rental. It's just me and my girlfriend. 

Orginally the house was listed for 105k, than 95k,  we offered informally 80k, owner said she take it. Didn't want to put in an offer officially until I did some research. 

House is small it has everything new. Leach pipes from septic, new electric, windows have been replaced in last five years, flooring. Comes with some nice furniture and AC. The owner grew up in this house and rented it for 10 years it's been on market for 4 weeks she doesn't want it no more, wants it gone.

The downside to me the garage isn't really the space I wanted to be it's a one car and roof needs done on it and even tho it's city water, it's on septic. 

How long does a septic tank last before it completely needs to be replaced? 

I imagine a tenant flushing down chemicals and other things that's is going to ruin the whole system seems like a recipe for disaster. 

I might put it in an offer for 70k but even then we're dealing with a small 2-bedroom house. Not sure yet.