Congrats on your new S5! You can use any standard micro usb charger with this device, but it won't change nearly as fast as the 3.0 usb (OEM) charger that came with your device.
It's definitely one of the things I love about this device. Initially I wasn't a big fan about them changing to this new usb size, but quickly got over that after a couple of days and seeing how fast this thing charges with the new charger.
Good luck with your new device and holler if you have any other questions or concerns.
Ya know I really haven't bothered to time them, but I do know it's pretty significant. I would suggest using a standard style usb cable for overnight charging, when you aren't in a hurry to charge your device, and the stock cable any other time. The great thing about the S5 is, you'll be hard pressed to drain this battery during a normal day. I'm a pretty heavy user and while I take my charging cable to work everyday, I rarely (if ever) plug it in because I usually have at least 30% battery left after a long day at the office.
Of course depending on your screen brightness, how many widgets you use and whether or not you keep your location services turned on (Just to name a few contributing factors) you should easily get 6 to almost 8 hours of "screen on" time with this device. Your mileage may vary of course, but once your device is set up and you're using it as you normally would, your battery should easily last 16-18 hours on average.
As for the 4G/wifi question: your data is used when sending & receiving sms/mms. I believe there are only a couple third party messaging apps that will transmit messages via WiFi only. I believe Hangouts just recently started allowing WiFi only transmission, but hopefully someone can confirm as I rarely use WiFi or Hangouts.