I showed up a little late to this party...
It's no secret I have much love for the BK9, but the 9 isn't going anywhere. The 4, 5, and 15 likely won't be available a year from now and the prices are too good to pass up.
If I were starting my Becker collection today it would go something like this: 4, 5, and/or 15 in whatever order you can as quickly as you can. I would follow those up with an 11, 14, or 24 then a 16/17 and a 10 working in spares of the 4, 5, and 15 when I can if I wanted to. Now that I have a very well-rounded knife collection for playing outside I can get more redundant starting with the BK9. Then the 2 and the 7 just to scratch the itch and a 3 if it interests you.
Of course the above doesn't account for the other models that are sure to be released while you build up your collection or for knives outside of the Becker line. I've also heard that the zombie line and most of the Johnson Adventure blades are on the chopping block as well so I snatched up a Potbelly.