my opinion
have the contract if possible broken into small peices that are independently prices out and are descreet tasks. If you give a maestro the whole job to start with disaster will soon follow. Make sure you know the materials that will be used, and you should be provided all facturas of materials. Finally make sure you have a plan that is approved by the governement, get a copy of the plan and if your not sure take it to a lawyer out side of the deal. AN official plan should have about 5 - 7 official stamps/seals on it (check in the legal section about this).
handing off the deal to a maestro is a recipe for disaster, especially if you put too much money up front, then the job will stop and they will say they need more money, never give more than the work outstanding, and be prepared to switch maestros if required. If you have some one that is recommended, that is your best bet.
A maestro might make . . . . well I don't really know, but I imagine if it goes over 10% of the costs i.e. more than $3000 of 30,000 your in definite trouble.
But don't base you decision solely off me, wait for other replies.
One thing to keep in mind is the work that is typically done is generic, the quality of maestros might vary, but a cement roof is a cement roof, is a cement roof, and if you stop a job before the roof is in place, any qualified maestro can pick up the work where it is left off, (something to keep in mind).
-Lee