First, Congrats on the Purchase of your new Truck.
$325 for a K&N intake is too much $, that is unless you don't know how to use a wrench, installing it is easy..it comes with complete directions. I paid around $215 for mine and it took me less than an hour Plus I Cleaned my throttle body while I had it apart.
I do all my own work, Brakes, upgrades, Flushes, changes, tune ups etc.. It saves a crapload of $ The only reason I sent it to my buddy was for tires and an alignment. Take advantage of whatever warranty or service plan you have on your truck.
For your exhaust, that doesn't sound like a bad price but what exactly are you getting for $425? Cat-back, true dual, headers?? it makes a big difference, If your thinking about headers and new Cats it should be alot more expensive, leave it to the professionals, they can be tricky to install if its your first time.
For your radio, if you install an aftermarket radio, they sell exact fit harnesses and adapters that directly plug in with no splicing needed. this way you wont fuck up your wiring. One end fits the GM harness and the other fits whichever model radio you purchased.
About lift kits, There is a member here
Storm442, He has a Nasty
2500HD thats lifted he may be able to give you some tips and advice.
Good luck and enjoy your new truck.
-Jimmy