Welcome to the forum...you made an AWESOME choice for your first Glock! My first was also a G21 (Gen3), which I recently traded to upgraded to a new Gen4 G21. You can't go wrong either way! Enjoy that big bore therapy!
I got sick four years back, had to sell all the guns I had to pay for the doctors bills. But I cold not part with my G21. Still have it now, and I am starting to shoot it again. Now I remember why I still have it. What a wonderful piece.