I started with Settlers 2, and IMO it was the best. I've played 3-5, kinda slacked off with 6 and 7. If you don't mind old ones, try this one out.
You don't have to do the actual Settlers 2, as that won't run on modern computers, but you can look for The Settlers II 10th Anniversary. It's a complete remake, exact same game play just with updated graphics and runs on newer computers.
Or you could just dive in with 7 and see how you like it, IMO if you start with 2, you'll see how it was originally meant to be.
3-4 stayed pretty close to the original, but after that they started doing it quite a bit differently, and I personally didn't like the changes.
I did a quick search, and you MIGHT be able to buy it from here. There is a demo, and that would give you a good picture of it, and perhaps take you into 7.