How do I replace a condenser fan motor in a central air unit?
First off, I vote for a new unit if you can swing it, but $100 is a lot cheaper and you are not likely to offset the cost of a new unit with the savings from a newer, more efficient unit for many years.

The condenser fan motor isn't that hard to change, you just need basic hand tools and a gear puller. It would also be a good time to replace the fan start capacitor as well.