It takes years to become very good at spraying, but if you get some tips and are careful, a fence shouldn't be too hard. Setting up the machine and cleaning it are very important and a decent amount of work. You will need to choose the right tip diameter and fan size, and the right filters. All of the machines I ran used three filters. You may have to thin the material as well, or run a decent machine and brush or back roll.
I used to spray elastomeric paint and that stuff had the consistency of pudding. Our big machines could run multiple guns and could pump that stuff, but the surface couldn't always handle a product that was atomized that coarse. We would spray it and then work it in with a roller.
Some of the fence products or stains are quite thin. If you spray a solvent based product you will need to clean the pump out with thinner. Paint thinner was something like $1.97 or $2.97 when I started in the business. Now it is $15 per gallon, and lacquer thinner is $20 per gallon. You will burn up easily a gallon or two to clean an airless, more if the hose is longer.
My little airless is a Graco 190es. It is the lightest commercial unit they had. It pumped up and down, so it could move more material than the smaller ones. The titan 440i, IIRC, is a great little machine as well. My uncle has a few of those now for his business, and one is dedicated to just clear lacquers.
If you rent a sprayer, it only has to work this time for you and the store is on the hook. If you buy used, you take the chance that it was not taken care of. Most people are bad about cleaning airless sprayers, especially pros. My uncle taught me to be very thorough. We always pulled the filters and cleaned them, and the guns and tips were always spotless. The machine will get paint on it, but you don't have to hang on to that while working.
If you don't want to do the job, get an estimate from a painter or handyman. It might be better than renting or buying an airless and learning to use it.
The big Speeflow hydraulic units are very good, but probably not what you would be looking for.