2012 Ram, right? I just did mine last night with that exact harness. It will come with plugs for 05-08 and 09+ rams. The plug with the rounded corners will be the one you want to use. Discard the other one.
If you have steering wheel controls you want to retain, there will be one additional harness to buy.
If you're hooking up a back up cam, this harness will generate the reverse wire through the CAN-BUS system as well. It alos provides a Retained Accessory Power so your new deck will stay on until you open your door, just like the stock radio.
As far as speakers are concerned, there are a lot of options. It really depends on how good you want to go. You could get some nice Alpine type R 6X9's that will work fine with or without an amp. Or you can get real crazy and do some 6.5" components in the doors by making an adaptor plate. Its all about how much you want to spend and what type of sound you're looking for.