I've shot and enjoyed the G21. It's an excellent pistol if you're looking for a high-capacity .45 ACP (and let's face it, who isn't looking for one...
). It's the same frame/size as my G20, which I really like.
Excellent reliability, great capacity. What's not to like?
All tritium sights wear out in about 15 years. It's the nature of tritium, with a half life of 10 years, you've got half the illumination at 10 years, and about 1/3 by 15. Glock sights are among the easiest to change out if you want new ones. You drift, or push, the rear and the front sight is screwed on. They aren't super-expensive, since they're so common, I think about $100 for a set. My 8 year old G20 night sights are still pretty good. Take a look at yours in the dark and determine if they're good enough for you.
Glocks are pretty hard to mess up. LE trade-ins tend to have lots of holster wear but few actual rounds through them. Great choice if you're looking for a shooter.
I would clean it thoroughly, replace the recoil spring/guide rod, and then don't worry about it. Even though yours likely has only a few rounds through it, I always change the recoil spring on a used gun. Glock guide rod/recoil springs are cheap. You can upgrade to a steel (or even Tungsten) guide rod, but it's not needed. I did that on my G20 to be able to have more consistency when shooting full power 10mm. But with .45 ACP, you're fine with the stock weight of recoil spring. I would just replace it with stock.
I've done the "25 cent trigger job" on mine, and it makes a modest difference. Basically, you're polishing the connector, safety plunger, and other trigger surfaces that slide against each other, in order to reduce friction. Trigger pull weight is about the same, but it's much smoother and more crisp when you're done. Big point is to keep the surfaces in the same shape/profile, but remove the stamping marks. It's a polish, NOT a modification, which could mess up the gun and its safeties...
ZEV and others make a nice trigger, I've tired out a couple of Glocks with teh ZEV and they're nice, but I've not bothered on mine. If you chose to modify it, then have fun, Glocks are easy to work on. A few pins, and a few springs. With the ease of modification and the number of available parts, they're like the small block Chevy's of the gun world - common, easy to modify, and fun.
I would get another magazine for it. 3 is my minimum number. Genuine Glock always work well. They're not expensive and they're readily available.
One final note - I don't shoot reloads in any of my Glocks.
The owners manual will caution you against this. The reason is the chamber - it's slightly unsupported, meaning that just a bit of the brass wall in the cartridge is not surrounded by chamber. That's why Glocks feed so well and are so reliable, that relieved chamber. If you get a reloaded case, that happens to have weakened a bit, and that weakened spot ends up unsupported, you can get a pretty good failure - which leads to firearm damage.
Lots of guys will argue this point, but I follow Glock's recommendation on reloads.