First of all, not every person reading your letter will be a male, so rather than "signores" you should put "Dear Sir or Madam."
"Presente" means "present," I believe? As in "Dear Sirs, I would like to present the following"? Perhaps you could tell us more about what it is you are stating in your letters so that we could better give you an English equivalent. In English letters, we would not start out with a single declarative word like "presente." Letters are tailored individually to fit the context. For example, if you are submitting a proposal, with prices and delivery time, to a customer, you might say:
Dear Sir or Madam:
We are pleased to quote as follows:
and then list the items, prices and delivery times.