Chesirecatdaddy's posts are on point.
Adopting from shelters isn't the way to go (in my opinion of course). Rules may vary from state to state but here, all adopted pets are spaded or nuetered. So no matter how physically or mentally impressive the dog you adopt may be, that restriction removes any possiability of you allowing it to procreate and pass on it's genetics. There are also health and socializing issues that come into play once a dog is spaded or neutered.
Secondly, to my knowledge, you will not recieve paperwork on your dogs (not that it really matters seeing as how you can't breed them) showing their genetic lineage. This basically means that you really dont know what breed the animal you are adopting is (no matter how much it looks like a paticular breed). This creates alot of variables, not least of which is breed specific traits that your dog may possess that you are not aware of (I.E. Human aggression). This is a big dice roll.
Thirdly, not only does whatever experience that dog went thru prior to being put in that shelter affect it's pychy (sp?) but actually being in that adoption place affects it too. How much so varies from dog to dog but trust, all of those dogs (outside of the ones born there or extremly young) are affected. If you don't mind getting into relationships that comes with baggage from previous relationship go for it.
In my opinion, your best bet is to research breeders on the internet, find a good one in your area and buy from them. I say in your area because, beside having the option to see what you're getting before you buy it, being able to pick your dog up after purchase is very important. Having them shipped on airplanes is not a good option.
What breed specifically are you looking to purchase?