I see that no one has jumped out here to help you.
Have you contacted Posado Don Rodrigo for a quote yet? I know the rooms aren't out of sight price wise the last couple of times I stayed there. The courtyard (s) offer so many opportunities for wedding venues that I think it would be a perfect location, if the price is right.
There are fantastic wedding planners in Guatemala that you should Google: I Do Guatemala. The two ladies that run the place are excellent and while they may be out of your budget I'm sure they would be nice enough to give you some direction.