There is not really any of it that you necessarily must have. A good router will run WiFi just fine to your Mac. Any external can be formatted to do Time Machine backups. If the machine you're Getting doesn't have a DVD drive installed, you may want to go ahead and spring for that when you purchase one, so all of your CDs and DVDs that you already have will be able to work with the machine.
Good point on the external CD/DVD drive...but there are as good/better models available and cheaper than the Apple drive. I picked up a blu-ray drive for about $30 online as opposed to the $70-80 Apple DVD drive.
Tappin and Talkin from my iPhone 5