If you are going to exclusively play zone, then I would suggest that you plan on taking a walk-on (at most 2). That will give you more funds to spend per recruit and won't hurt your depth chart at all. You really only need 9 to play zone (maybe only 8). Also, holding back some funds to vulture any late-drop transfers available just before signings is a great idea. However, you might think about browsing transfers & calling coaches of JuCo players to see if any already know flex/zone. You might find someone that is worth starting for a season or two that could really help you be competitive this season.
As for targets, here's what I have learned: post players need to be very strong in REB & BLK to play effective zone defense. In theory, the flex should require P and PE in virtually every position (as opposed to BH), but I don't think the simulation really demonstrates a significant difference between flex and other offenses in that regard. Which is to say that you need guards with very good PE, BH & P to run the offensive effectively (and survive the onslaught of pressing teams that you face in Wooden).