well the Apple 3G dock has a line out, line outs are ideal for sending the audio signal to another device... say a stereo or computer speakers. if you try to plug headphones into that though, you'll find the signal will be very low almost inaudible... that's why i say you'll want to just plug your headphones into the iPhones headphone jack at that point. again unless $30 is more than you wanted to spend, the offical Apple 3G dock is really good build quality and very compact, not worth $30 but nice none the less.
http://store.apple.com/us/product/MB484G/A?fnode=MTY1NDAzOQ&mco=MjE0NjMxOA
newer batteries function better than the old ones you're thinking of. if you're planning on leaving it on a dock for 8+ hours, you should consider letting it discharge all way to powering off, maybe once a month. other than that 8 hours on a dock shouldn't do much if anything to shorten the life of the battery.