The question, "What other calibers do you already have?" is a good one. If limited to only a certain number of weapons, I would suggest also limiting one's caliber choices. So instead of spreading out and having every handgun caliber out there, concentrate on multiple guns in just a few calibers.
I, for one, totally skipped 40S&W. It just isn't for me...
45acp, 40sw, 9mm, 38spl, 380acp, 22lr
12ga, 5.56, 30-30win
2 Chron. 7:14 Christians must seek His face