Henrik, you don't have to confine your agent search to one in the Miami area - any agent in the US who is willing to work with you will probably do, though you should probably look for one who is a member of CLIA, and perhaps also ASTA or IATA. Someone with CLIA membership is likely to be familiar with cruising, and an agent who has experience working with non-US customers would be an extra benefit.
Malcolm is right about saving money buying your air tickets separately from the cruise line (being from Europe, you may need to do that anyway), and also about planning to arrive the day ahead. You have an advantage in that you gain time coming west, so you can pick a flight that arrives in the U.S. in the morning and have enough time to get to the port your cruise would leave from (Miami or Ft. Lauderdale, for example), and have time to rest the night before your cruise.
The advice of paying with a Visa card is also good,for protecting both you and the agent you would work with, as well as making payments easier - no international payment coupons needed, etc.