What is "very old?" If it's fifty or sixty years old or more, it will probably be very easy to work on and get it running as well as it did the day it left the factory.
It's not hard to learn how to do it, and will likely cost a lot less than a new sewing machine.
That said, I had a Brother CS 6000i which I just loved. It was a pretty sturdy and capable machine for the low price and very easy to operate.