Depending on what model of grill, a standard 3 burner is what you will likely be looking at. Weber didn't change their design too much over the years. Take the grills out and look at the flavorizer bars. If they need replaced, they roughly cost $35 to $50 depending on which you buy. That’s a common replacement for any grill.
(The actual grill plates are second.)
Remove the flavorizer bars and inspect the burner tubes. Check for rotting or damage to them. There is a small crossover tube that is used to transfer the flame to the others when you first ignite it. They can break or rust out.
You should fire the grill up with the bars off.
Check the flame to make sure it looks even. No plugged up holes.
Also check the propane hose regulator valve assembly. Make sure it’s the newer OPD style if its an older grill.. You don't want the older tank set up if posible. It can be updated, but another cost you may not want.
Other than obvious cosmetic, there isn’t much to a gasser. Frames can rust out if in a harsh climate. Also, the firebox could get warped if someone got it too hot.
Most gassers can be rebuilt for under $200, but it all depends on how much you spend on replacing the grates. SS or Cast iron cost more.
You don't really want the cheap grill plates. They make your grill go from basic to awesome! The same way a 24" monitor is superior to a 15" monitor. lol..
I love my cast iron, but others rave about the SS grates. You usually won't find either on a basic grill. I recommend you go to your local big box store and inspect a new one first.
It will give you an idea of what you're looking at. Beware; looking at new grill can give sticker shock.
Good luck with your hunt and unless it’s already a good deal, don't be shy to negotiate a price that you are comfortable with.
Here in the Bay Area, they practically give them away!
Hows this one?
Red Gasser