We were told we had a septic tank when we bought a house. It needed emptying about every 4 - 6 weeks, so was not a septic tank. However, when someone inspected it, there was a chance it had a filter in so then it became a septic tank again. Then we were told that the seller had used a pump to syphon water off and into the ditch, so it became a cesspit again.
We never found out what it was but the next buyer replaced it, which we would have done if someone had been honest enough to tell us if we needed to.
Be sure of what you have. Be aware that if it needs emptying you will not be able to let anything else into it without the threat of unpleasantness and that when it is emptied you will need to close the windows because it will pong. So will your neighbours tanks.
We ran water out of the bath/washing machine etc. to water the garden and save a week or so on emptying, it was expensive.
With the oil heating, do not let the boiler run out of oil, it will stink and can be a pain to restart. Watch your tank level and keep an eye on it if it is accessible to others because they get drained by thieves.
Country living is not as simple as it might be.