18K feet is a long way. If at all possible I'd try to find someone in your neighborhood that has it and see what sort of performance they have. Your DSL company might provide you with some referals, or they might not.
DSLreports.com will tell you what sort of success other customers in your town have had, but unless you know exactly where they live that may not help you much.
Talk to the guy who actually hooks up the phone line if you can. Depending on your situation he may not have to visit your house, so this may not be possible. And if he comes out you may not even know it. All his work will probably be outside, some of it down the street. So you may need to stay home all day and peek out your windows to catch him.
More often then not he won't actually work for the DSL provider directly. He'll be a phone co employee or a third party tech. He'll know more then the reps you talk to on the phone, especially about how things work in your area. And he won't have as much incentive to gloss over the bad stuff.
Definetly read the fine print very carefully. Most, but not all, DSL service has a contract. Most that do have a contract have a period of time after installation that you can get out of the contract if the service isn't living up to your expectations. If yours doesn't, DON'T BUY IT! But those do have an out period will have stipulations on how long you have to make up your mind. Make sure you understand what those stipulations are in your case. Get assurances in writing.
Record the names of everyone you ever talk to on the phone, time and date, and what you talked about. Make sure you ask them a couple times during the conversation if they are putting everything into their computer that you are talking about. Your number one greatest help in getting any service disputes resolved succesfully will be a detailed record in the companies computer system of every problem you've ever had and any promises they have made to you. Without that record for them to look at, you are always on your first call. And call centers rarely do anything on the first call. (I know, I work for one.
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You almost always will have a choice in ISP's, but regardless of ISP, it will almost always be the same wires and technicians working on them. Changing ISP's will likely not improve your speed, and probably won't improve the reliability much. But all ISP's are not the same when it comes to customer service. How seriously they take your issues, and how hard they will try to make you happy when things break.