I hate house with old septic tank.

Check if there are large trees growing around drain field area. If there is, be cautious. One of my rentals had flood due to drain issue. I finally changed to city sewage. Besides construciton and replumbing fee there is connection fee charged by city. It could cost a big. Some cities won't allow rebuild of septic tank if there is sewage connection available. 

Once there is slow drain problem, it is a few hundred $ to pump. You can not wait for even one day. Could be stressful situation. I have done caustic treatment, pumped numerous time within one year before I decided enough is enough.