It really depends on what you need (and want)
You will likely hear similar recommendations from other users but Ill throw in my two cents too. The answers to most of your questions really depend on how you will use your iMac.
I do a lot of photo/video editing and I multitask like crazy... Its not uncommon for me to be editing, chatting, listening to music, working in my e-mail and surfing simultaneously. For me the larger screen means more real estate for my tasks. If you just use the iMac for singular tasks or if youre never going to watch movies on it from a distance then you may not need a larger screen.
As for buying a new iMac versus a current model... Again, it depends on what you need but also what you like. If you use it for demanding tasks such as video editing, gaming, etc. then you may be better off buying a new iMac with some upgrade options; however, if youre primarily going to use it to check e-mail, chat and surf then a current model will suit you just fine.
For software it again depends on what you need. Ive found that many users can get by just fine with whats included with the iMac. If you like to do basic stuff with photos/video or if you need to do word processing, presentations, and spreadsheets you can simply purchase iLife and iWork. I have both Microsoft Office and iWork... No particular reason other than it depends on which interface I feel like working with at the moment. If your work requires the heavy duty software then you likely already know what what you need... possibly Photoshop, Premiere, etc.
Paying less than retail... Shop for a refurbished iMac or wait until someone upgrades from an iMac that will suit your needs. I can imagine that when the new iMacs finally start to sell that you will be able to find some really good deals on the current models.
Hope this helps... Welcome to the iMac!