To the OP, as others said, if you like the way the G21 feels in your hands, go for it. Glocks aren't for me, but they are an excellent choice if they work well for you.
As for caliber, pretty much everyone is reporting that .40 S&W is by far the easiest to find center fire caliber these days. Even if my local Wally World is completely wiped out of all other ammo, they usually have 30 boxes or so of .40 on hand. 9mm is relatively easy to find in my area. .45 ACP is almost non existant however, and price wise it is starting to approach the territory of becoming a reloader's round (for most of us working stiffs) like 45 Colt, .44 Special or 10 mm.
As for a 22, I wholeheartedly recommend purchasing one first, or a close second. With ammo prices and primer unavailability, I find myself shooting about 25 rounds of 22 LR for each centerfire round. Since you are going semiauto for your centerfire purchase, I would recommend a semiauto 22. You can find good deals in used Rugers and Browning Buckmarks.
For instance, my sister picked up this very nice used, but like-new Ruger Mark II with adjustable sights and 5.5" barrel for $199 about a year and a half ago, and that was during the peak of the gun buying panic, post Nov '08 elections.
Come to think of it, if you take a little extra time and buy a
used Glock and a
used 22, you probably wouldn't spend much more (if any) then you would buying a new Glock alone.