I do think you need to be leveled up a bit since the letter from the Jarl mentions he's heard of your efforts. Level 10-ish maybe?
I buttered up the jerk of a Jarl of Falkreath with some Black Briar Mead, then killed some bandits. After that I can buy property - offered at 5,000 gold, when I was just level 8. Once I get out of the dungeon I'm in (WAY over my head, so fun) I should be able to afford it.
It's been fun spotting bits of new items in the world. Kids clothing, sacks of flour, bales of hay, stone quarries, items at the forge. I forgot how hard I get sucked into making a new character and developing them.
I'm playing a female Breton, an unusually hostile, vegetarian pure archer/sneak seeking a homestead. She'll knock off the achievement for 1,000 gold bounty in all holds for me.