Your best bet is to save up the gil for the small and wait for a plot to be auto-demolished (timer starts when 3.4 hits). Just keep an eye on the ward listings once and awhile and if a plot has opened up it will be a white price next to it instead of a name. As someone can't sell you the house, they can only demolish their plot so that you can buy it. You'd be paying double the price (at least) in order to get it from someone and on top of that the GMs discourage that type of activity.