It is a myth that shelter dogs are "sick and dirty and have a lot of disease." I have a real sweetheart of a shelter dog and she is clean with a nice, shiny coat, even if I haven't given her a bath yet since she's been here on June 4th. Matilda is also extremely healthy; moreso than the Scottish Terrier I had which wasn't a rescue. He had thyroid, heart and skin problems which probably all played a part in him leaving me too early.
Rescued dogs have accomplished a LOT, and here's just some accomplishments they've made at the top of my mind:
Many rescues are serving as registered therapy dogs, assist in Search and Rescue and even help the highway patrol. Some are bomb and drug detection dogs. There are rescue dogs who help with deaf people, and there's others that assist their owners in wheelchairs. Some are even used to sniff out termites and mold, saving homeowners thousands of dollars of damage. There is a top sled dog racing team made up of rescues. Some of the top agility and flyball dogs are rescues. The dogs that played Benji? Rescues. Pal, the first dog that played Lassie? A rescue, with papers, even.
http://ramonahumanesociety.org/myths.html
http://www.petlvr.com/blog/2008/06/top-five-myths/
http://www.rescuers.com/rescue_myths.htm
http://www.jamaicadogsled.com/
http://www.lawdogsusa.org
Also, getting a dog to show off to your friends is the wrong reason to get a pet. No responsible breeder will ever place a dog or puppy with you for that reason. I will echo the statement of others and say that you aren't ready to have a dog until you get rid of that stigma against shelter animals, and know how to research the breed and proper reasons to have a dog.