Applecare can be worth it, but you don't need to buy it when you buy the iMac. As for saving $300 by buying it on eBay, seeing as it costs $169 for an iMac, I'd take that price with a pinch of salt.
RAM used to be hideously expensive from Apple, but is now about the same price as from elsewhere. Get it from Apple if you don't like opening your computers.
As for the mouse, you may like the Apple one, but this is something you can decide after you've tried it for a week.
Software? Get Launchbar. If you're doing Web work, get Coda. Try the iWork trials on the Mac and if you like them, buy them. Text editor? BBEdit. You will need VLC and Handbrake if you're ripping DVDs and to round out the video playmack experience, get Perian.
On top of that lot, the professional software you should get depends on what you want to do with it.
Spec? That depends on what you can afford. You need the 24" though.