Eyota just posted this but because its such good info, reading it twice wont hurt:
Have you been around dogs before? Why would you believe a dog would automatically be agressive to you our bite?
First of all, before getting this dog, you should really research the breed some more. All breeds have characteristics that are common to that breed - it's why some people have favorite breeds and own only one or two kinds of dogs.
You also want to know more about the breed so you can decide and be sure that this particular dog will fit your home and lifestyle.
For instance, if you don't spend much time at home and think the dog will spend the majority of time outside in a kennel, then you shouldn't get a Chihuahua or even a Great Dane. Both are short haired breeds that don't tolerate extreme temperatures very well. Both are also relatively "needy" breeds that need to be in the house with their families because they are emotionally attached to humans and don't do well without that daily, constant contact.
If you have little children who like to run and scream, you probably don't want a herding breed like an Australian Shepherd or Sheltie - both of which would chase your children, nip at their heels, and possibly knock them down. This is what they were bred to do (with sheep), and it is instinctual to chase and try to herd running beings!
You can go to the AKC website - www.akc.org - and learn more about all sorts of breeds and the characteristics, both behavioral and physical, that might better help you determine if this particular dog would fit your home.
You should also talk to this owner a lot. Find out why they are giving the dog up. Go VISIT the dog before agreeing to take it. Ask to see it's vet records. Ask if it's had any training (obedience classes - I recommend a beginning obedience class for ANYONE who gets a new dog - so that you two can bond and learn more about each other AND learn to train your dog to behave).
After you've visited the dog, go home and think about it. Decide if you have the time and the money to care for the dog properly. Will he be alone for more than 10 hours at a time? Then don't get the dog. Do you have enough $$ saved up so that if your dog becomes seriously ill, you can take it to the vet? If not, don't get the dog. Can you afford an average of $100 a month to feed, care for, and otherwise keep the dog? Then don't the dog.
And we haven't even gotten into behavioral issues yet!
But seriously, many dogs will be cautious around new people. That's not unrealistic. But I would NEVER take a dog that tried to bite me or was aggressive.