Re: glock 21
Sounds like you got a slim frame, which will be more comfortable than the regular frame for most people. Adjustable sights are nice if you're going to be shooting for fun. The more mags the merrier has always been my motto, so four is a good start.
It looks like the new Glock 21s are retailing for around $570 or so, so 2 extra mags and adjustable sights for $20 less with a small number of rounds fired sounds like a decent deal to me. My first handgun was a used Glock 17. It came with factory sights; 7 mags; extended mag release; mag well plug; steel pins; original case, shell, and paperwork; and the barrel had been polished. They said it had a few hundred rounds through it, but it was clean on the inside. I walked away with it for $475 and have never had a hiccup with it.
Don't worry about the plastic guide rod. If it breaks (it most likely never will), get a new metal one for less than $30. Don't worry about replacing anything on it unless you find you aren't happy with its performance. Otherwise, leave it alone and have fun shooting the piss out of it.