My understanding is that it was illegal in Haiti for foreigners to buy property in Haiti until the (illegal) U.S. occupation under Woodrow Wilson - the U.S. "gave" Haiti a new constitution allowing foreigners to own land, stating it was good for them if foreign investment could come in. This of course, has generally not been the case as mostly all it's done is create sweatshops that pay 20 cents per day.
It has helped a little - allowing digicel, free hospitals (HAS and Paul Farmer's), power companies and other things Haiti did not have the means to build on their own. But mostly it has just created extremely cheap labor for greedy companies.
Thus it is possible, but very complicated, and I don't know the whole process. Also as warriorbjj stated, one of the hardest parts is figuring out who owns the land and who is selling.