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, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
2 Chron. 7:14 Christians must seek His face