I'm in the same boat. So far all I have is the Cleaner. I'm waiting on a Pred 2 Battle-Damaged and have the Pred 1 preordered. Start with the main guys. Even if you don't end up with a huge Predator collection, these would still look great in an overall "movie/sci-fi/whatever" collection, know what I mean?
Personally I think that's a great start, and I'll probably hold off a bit - three main Preds is good enough for the time being. I wouldn't mind a Scar to finish off the main movie Predators, we'll see.