Customizing your iMac is little different than customizing any other computer. There aren't any 5K-specific tools.
Top on my list is BACKUP! OS X comes with built-in backup software called Time Machine. Get yourself an external hard disk, plug it in for the first time, and you'll be asked if you want to use it for Time Machine. You'll be up and running (and protected) in a matter of moments. After it's setup, leave the drive plugged in at all times. If you want an external HDD for other purposes as well, I recommend using a separate drive - backup is too important to mess with.
If you prefer a different mouse (as already mentioned), get one. I happen to like the Magic Mouse that comes standard - perhaps my favorite mouse ever (but I'm a "love the one you're with" kind of guy).
Other people prefer Apple's trackpad to Magic Mouse. I think part of that preference comes down to whether you have been using a laptop that has a trackpad. I've barely used a laptop since 2010, so I'd need to re-learn trackpad.
I do prefer the Apple USB keyboard to the wireless that comes standard with desktop Macs - I like having a numeric keypad and would rather not have to change/charge batteries. And every so often, the USB keyboard's built-in USB ports come in handy. (I prefer a wireless mouse, though - the wire interferes with movement.)
If you get a non-Apple wireless mouse or keyboard, get a Bluetooth model that will pair directly with your Mac. In my experience, the wireless mice that come with USB dongles are more trouble and waste a precious port.
I also have a SuperDrive external DVD drive. I haven't installed software from a DVD in many years, but I do have a library of music CDs that I needed to copy into the Mac.
After that....I have extra cables for my iPhone and iPad - better to have dedicated cables for syncing than switch them back and forth from the charger - and it's always good to have a spare.
If you plan to use FaceTime and/or Skype, consider a USB headset - while the built-in camera and speakers are fine, it's usually best to have a microphone right next to your mouth.
Printer? Just about any printer that connects to wi-fi or USB will do just fine, but if you're getting a new printer anyway, look for one with AirPrint - they're even easier to setup, and you can also print directly from an iPhone or iPad without an additional app.
Cool features (to me) that don't require a purchase: If you have an iPhone, SMS forwarding to your Mac (your SMS messages show up in the Mac's Messages app). Also, FaceTime > Calls from iPhone (you can answer and place phone calls from the Mac). Overall, I love the iOS/Mac/iCloud ecosystem/integration.