It is an intimidating job when you do it for the first time, but when you've done it once or twice, it is pretty easy. I would be inclined to do both sides, as the wishbones are the same age, and the other is likely to go in the near future - but you don't have to do this.
Two words of warning:
- When doing up the 4 inboard bolts (which is fiddly), make sure they are not cross threaded. Get them in, and have several turns with fingers only before starting with the sockets. If you cross thread, your new arm is scrap.
- Take it for 4 wheel alignment afterwards, it will be all over the place after doing this work. This is the reason I tend to do the whole lot in one go - only one trip to get the wheels aligned.