I have just purchased an old farm house that has a septic tank. The tub drains slowly and the toilet will not flush toilet paper or waste. The toilet will flush if you pour about a quart of water in the bowl as it's going down; then it will empty out.
I would locate the septic tank, have it pumped out, then call a plumber with a snake and camera to check out the piping from the toilet to the septic tank. In my area a septic system is subjected to a rigorous test to see if it works properly. If not, it has to be fixed before the sale or can be sold with an "as is" sales agreement with the prospective buyer.