I just picked up a 1991 Glock 19 from a client. Other than a thorough cleaning, is there anything else I need to know about the gun.
Disclaimer: I'm a huge fan of Glock 9mms; not so much the other calibers. I had a G19 from about that time frame, and I used it hard for about a decade before it got traded-in for a newer model 17. Never had any problems or complaints with it. None. And I shot it regularly (averaged maybe 2000-3000 rounds of full-power ammunition every year).
The G19 is often considered a perfect compromise between shootability and ease of carry. I think it (along with the same size .40 caliber G23) are the most common choice of the one-Glock gun owner.
If it only has the original magazines, I'd recommend getting some new ones. The mags have been greatly improved over the years, and although the old ones would work fine with many hollowpoint loads (assuming the springs are still in good shape), the newest ones will work better/best, as well as being more likely to drop-free of the gun when the mag release is pressed (old ones were actually designed to stay in the gun and be pulled-out manually). If the old ones are still working well, you can keep them for target work with RN/FMJ loads, but I'd employ newer mags for any serious uses.
The sights are often considered the weak point in the basic Glock design. An upgrade from the stock plastic sights to metal ones, even glow-in-the-dark 3-dot nights sights like many police departments use, is a popular and useful add-on. Glock can do this for a fee (and will inspect your weapon and replace any worn parts while it's at the factory, for free); call Glock's customer service number for more info. Most gunsmiths will replace Glock sights also, but the charge will certainly be higher (maybe 2x or 3x as much as the factory).
To disassemble a Glock, you first have to pull the trigger to dry-fire it (this takes the spring tension off the striker/firing pin). If the gun is not carefully checked and cleared of all ammunition before EVERY disassembly session, you will put a hole in something (or someone), eventually. Get in the habit of checking it every time you pick it up, and double-check it before disassembly. Get all the ammo and loaded magazines out of the area/room where you will be working on it. This is actually common sense for disassembling ANY weapon, but the Glock design makes it particularly unforgiving in this area.
Enjoy your new-to-you Glock!
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)