What condition is the existing fence and paint? What type of paint was used on the fence before? How often do you want to do this again in the future? As much as you don't want to do it now, doing it again in five years is going to be even worse if you do not do all the needed prep work first.
I'm not a fan of pressure washing anything wood before painting it. This Old House TV show on PBS had an episode where they pressure washed the siding on a wood house and then came back a month later and found that the wood was still wet inside, between each other. It wasn't 100%, but there was enough moisture that the painting guy said there was no way he could paint the house with it like that.
I'm a huge fan of using high quality primer before painting. If you have bad spots, use something designed for exterior wood. Sherwin Williams is my go to store for anything that will need special attention.
Spraying is probably the easiest way to do this, but you will have realize that a lot of the paint will just go off into the air. Another problem with spraying is that you will miss spots if the fence had irregular angles, or a rough surface. In my experience, I will spray, then go back over it with a brush or roller, depending on the job and what I'm spraying. I've also found that sometimes a brush is the fastest way to get it done, even though it might not seem that way. I factor in half an hour of cleaning the sprayer before deciding to use it when debating on what to use.
