1. Exactly how important are stats like ATH and DEF to the zone? Which stats are the most important to have (ATH/DEF for bigs, SPD/DEF for guards)? ATH and DEF are very important although the DEF doesn't have to be at a M2M level. Also, as guyo said, REB and SB are essential for your bigs. A STA of 75 will allow a player just over 24 mpg at a FF using Fatigue setting.
2. When is it better to switch to a 3-2 as opposed to a 2-3 with a + setting? I only use the 3-2 setting 2-3 times per season but other coaches use it quite heavily. I use it when my opponent has 3 or more strong 3-point shooters. If he only has 2 strong shooters, I will play 2-3 at either 0 or a +1 setting.
3. Basically anything else that would be important to consider in recruiting and gameplanning with the zone? Since Zone allows you to play with a shorter bench, it is not uncommon to take 1 or 2 walk-ons and use the extra cash to go after better players. Zone players tend not to get in as much foul trouble, however, they will also create less fouls for you opponents.
4. Can a zone defense compete with a top-tier D1 M2M defenses? I've seen good teams run a zone, but is their championship percentages equivalent to the percentages of teams that run zone (ie 10% of teams use zone, 10% of championships were won by zone teams). I ask this because I'll most likely need all the help I can get, and if I'd not play a defense which handicaps my team from the start. I won a DI NT with a Zone and other schools have also, so it can be done. I have no idea about percentages but a quick search showed 5 of the Top 25 DI teams in Iba play Zone. However, only 1 of the Top 25 DI teams in Knight play Zone. You can draw your own conclusions from that.