Regarding screws: I have a no-CD LC 550 which has its original packing list. 2 screws are mentioned on there, but they're practically the only part missing. I had to put two of my spares on myself since the tabs are broken. Aside from that and some yellowing though, the computer is in immaculate condition, albeit with brittle plastics...it's re-capped and I'm actually going to be selling it this week if anyone's interested...so keep your eye on the trading post!!
I've had an LC since it was new and there was no screw in the box.
On the topic of LC durability, the Classic, LC, LCII, LCIII, IIsi, SE/30, and SE are the tanks of the group. None of them even have brittle plastic.
However, I had an LCIII with the newer case lid completely destroyed in the mail when I bought it in January. I'm still trying to figure out if I can just epoxy the sides of another LC case to it (which has a damaged front because I accidentally left it in front of a heater for too long).
Color Classics don't hold up well either. The one I'm grateful for coming in one piece is probably my PowerBook 190. The plastics on it are perfect as of today. Not sure what they'll be like in 5 or 10 years...I'm keeping it in a climate controlled environment (my apartment, but it just seems lower grade than the SE, the LC, the IIGS, basically anything older than it (or even the regular 100 series).