You should be able to talk to the Jarl of Morthal and ask about buying property, whereupon he will tell you there are no houses but there is land and to speak with the steward.
Just remember what so many other people were forgetting (or not paying attention to): it's the *steward* who sells the land. Not the Jarl. Not the housecarl, not the dishwasher or cook, but the steward.
thanks. I recieved a letter from the steward of Riften informing me I could adopt... but I couldn't cause that old "You kids will never be adopted" witch was still alive and I had no adoption dialog despite Honeyside basemant being turned into a kids room.
So I went to Morthal having recieved no info/letter from anyone and went to the steward and he had dialog "I would like to buy a house" which he responded as you mentioned "There is no house available but I have a plot of land for sale" response. I bought it and now have a fully funished home outside Morthal.
I also went and killed the old witch in Riften and the young woman there let me adopt a little girl. Already gave her a toy doll and two dresses. Though I now have a feeling I'm about to be kidnapped in my sleep cause a courier met me outside the orphanage with a note that said "We Know"