As a general rule of thumb, one litre of paint will cover eight square metres with two coats. This is for a flat surface and you should expect to use slightly more on a fence. Stain and oils can be a bit more variable because the timber will soak in more or less depending on how dry or thirsty it is.
Fencing timber will suck up quite a bit of paint, so you should expect to use more. One litre per four square meters wouldn't be unusual. Unless you are doing one exceptionally thick coat, you need two of them instead. It will depend on what colour you are using, but I'd suggest two coats will be most practical.
The best type of paint is the one that will last the longest, is tough and hardwearing, and has the best UV protection. My recommendation has always been Dulux Weathershield as it has outstanding characteristics and is designed to withstand the harsh Australian weather conditions.
Dulux Weathershield is guaranteed to not flake, peel or blister for as long as you are living in your home. You need to follow the directions for this warranty. I'd imagine you might want to paint again in around 8 years to keep it looking fresh.
You'll definitely need a roller, and I would use one with at least a 10mm nap, as this allows you to get deep into the timber fibres. A brush will also come in handy as there will be spots that you can't reach with a roller. I've always purchased the cheapest brushes and thrown them away afterwards. However, recently I purchased a mid-range brush and have been washing it out when I'm done. It makes life so much easier, increasing the speed at which you can work as it holds the paint in the bristles much better. Next, I'll be investing in a premium brush. - MitchellMc
You need 1L/8sqm for 2 coats. What you'll see on the tin is 1L/16sqm - just remember that's for 1 coat.
Painting is generally an inexpensive option so you shouldn't find it costs you too much. Just remember you need to prime it first if you're painting it. There are several 3-in-1 and 4-in-1 options so just get some guidance from your local Bunnings store on which would suit your needs.
I always recommend Dulux Weathershield for exterior applications because it's high quality and I've never had any problems with it.
Depending on your budget you could always consider some sort of screening / cladding. Being a timber fence you'll find it easy to fix the panels to it. And remember, you don't have to do the whole fence in panels. Just a few matched with the right colour paint of the fence can look great too! - ProjectPete