Lower Ball bearing
 

I have been hearing squeeky noises coming from the right side of car
and I took it to the mechanic. He said the lower ball bearing was bad.
When I push hard on the right side front fender i hear the squeeky
sound more. He said I should change both ball bearings. I drive about
80 miles a day.
My question are:

What does the ball bearings do?
How long more can I keep driving it before I fix it.
Is this a big job?
He wants $200 for the job? is that fair? its NYC.
whats the worst that can happen?
Is this a job I can do myself?

Thanks