The current owner has the bulk of the responsibilities. Yours fall into a couple of categories:
1) What happens if he can't deliver? Permitting, Construction delays, etc: Make sure you're covered.
2) What are you buying? If it's not in your contract, or referenced somewhere, it's optional. He could decide to not do it, alter it, charge you for it, etc. It sounds obvious, but its not.
3) Understand the other units and the owners role: will he be contributing to the capital reserve while the other units sell? Is he the voting majority? Can he rent the units? Again, be clear about what his options are, and get it in writing!