Go for the airbags, you would be surprised how much the truck will sag without them... Also, get a shift kit (preferably Trans-Go), it will help your trans last a LOT longer. The massive low end torque the CTD makes WILL destroy a 47RE without a little help.

Now, if you decide you want more power, 300rwhp/700ft-lbs of torque (vs 140/310 stock) is easily attainable with mostly FREE mods. The only thing that will cost a little money will be gauges, aftermarket filter, and a boost elbow; the rest is all labor that you can do yourself... BTW depending on how heavy your truck is that can get you into the 15s in the 1/4 mile (my truck runs 13.4@102 with a slightly larger turbo and bigger injectors)