In reply to Akers:
Firstly, you might consider checking out tricams. Cheaper than friends and will go into places that friends won't. Slightly fiddlier to place, but they don't wander like friends do and when you have one well placed it's a lot more confidence inspiring than a spring loaded device. That's my opinion anyway, it's by no means universal
One cam on its own isn't much use, so if you're going to be buying them piecemeal, it will be a while before you have a useful selection. When I bought mine, I chose 4CUs from needlesports, because they were cheap and I started in the middle of the range and worked outwards in whole sizes, ignoring the smaller end of the range [below size 1] I think it took me about six months to accumulate the whole set.
You're the best person to judge what sizes to go for at first, since you can see the climbs you'll be using them on. it doesn't matter too much where you start. You'll have them a long time and sooner or later they'll all get used.