In my limited experience there was no standard way of doing things with old systems. In my most recent experience the septic tank consisted of an approximate 150 gallon earthern floor "tank" made of cement block with a tile line that went down to a lake. Other older systems could have nothing more than a buried barrel for a tank. In many places it would not be legal for a seller to sell a property without a conforming septic system. I would plan on replacing your system and would adjust your purchase price or make the seller upgrade the system. If you don't you should plan on paying for it yourself if you want to rent or sell the property.