It is a misconception that you can go without DEF in a zone. The one way that is true is that a bad defender could be balanced off with a great one and, with a high zone IQ, he will foul out less than he might in other defenses. However, your opponent's FG% will be higher while he is on the court.
The computation is grouped by position. So, in a 3-2, the C & PF operate on the same equation (which, I assure you, focuses on ATH, BLK & DEF). Likewise in a 3-2, the equation for PG, SG & SF is identical. In a 2-3, the PG & SG use one calculation, SF & PF use another, and C is on its own (desperately needing ATH, BLK & DEF...and REB too...for the rebounding calculation). If you think of defensive rebounding as part of defense, even though it's a separate calculation in this engine, then REB is an absolute must. Zone is the worst defensive rebounding system (which the engine gets right...). So, you've got to have REB 3-5 whether you play 3-2 or 2-3.
What does that tell you? Like every other defense, guards (1 & 2) do not really need BLK or REB. Your 5 simply must have BLK & REB! If you end up with a couple great PFs, then you could play 2-3 all the time and play the one with better handle at the 3. If you end up with a gang of guards, you might try to make sure you get several C's to play at 4 & 5 and play a 3-2.
The big tradeoff is that a zone defense will foul less, requires less stamina, but will generate few turnovers, and is handicapped on defensive rebounding. So, it is tempting to maximize guard speed and big rebounding/blocking to offset some of that.