Ring shank nails are great for the first few years, but since each board is just a little different then the other, you will find that some nails hold wonderfully, while others get loose. It's the nature of wood and nails when used outdoors. I will never rely on nailing anything that is exposed to the elements.
If I was building the fence, I would use screws with the star drive. Currently, DeWalt has the best driver tips that I've come across. They are silver in color with a yellow stripe around them. I normally do not like DeWalt tools or accessories, but my Home Depot guy talked me into giving them a try, and he was right. They really are better then anything else out there.
Then get a good quality impact driver and two sets of batteries. There is no wrist strain with an impact driver, they just make a lot of noise, but they take zero effort to use. I personally like Makita the best, but anything else in that price range is probably alright. Cheap tools are cheap for a reason.
Besides holding better then nails, you can adjust a board with screws, pull them together and best of all, undo them if you see that you've made a mistake. Using ring shanks on new wood will just about guarantee destroying the wood trying to get them out.
Since you are spreading this out over several years, it's also easier to just grab some screws and the impact driver to get a little bit done at a time.