I bought the 20gb model 2 months ago.
Re your Qs 1&2 - I think the 30gb model is no longer produced and hence does not appear in the web page and is discounted.
Re your Q 3 - The dock is nice to have but not essential as you can just plug the iPod into the firewire cable on your desk. I have found that the remote is really convenient - when I am walking around or on the bus, I can adjust volume, pause, and change tracks without unzipping my coat and pulling my iPod out of the belt holder. More important is the extra HD capacity. I have found that I really need the room for my music collection. I struggled with the price/value ratio with options at the time (10/20/40gb). The iPod is such a great product that no matter which one you buy rest assured that the $ is well spent regardless. If you might need the space, pay the bucks for the 20gb, you won't look back.
Re your question 4 - The new 15/20/40gb models are all 3rd generation. I have read that the 30gb is 3rd generation as well but is no longer built - someone else may have a better sense of this.
By the way, you should have posted your question in the "Peripherals" Forum - the moderator may end up moving it over ...