Put the amp and subwoofers to the back burner.
Get the head unit working first in that sequence.
BEFORE you touch the head unit - and I know this isn't sexy or cool or sick or anything - make DAMN SURE THE BRAKES WORK!!
Be sure your brakes work.
Can I mention, you should be sure your brakes work?
After that, a good tuneup to get it running as good as it can (for a 5.9, that's cap, rotor, wires, plugs, air filter, PCV valve - get OEM factory if possible, try to avoid Fram unless you like buying oil), PCV breather filter. Don't buy the hype - for a stock motor, there's no real difference from new copper and new E3/Bosch Plus 4 (NONE of those that swear by the E3 or Bosch Plus x plugs have compared new to new; they compare to the old worn out plugs. Crap, almost ANY plug would outdo some of those old plugs!!).
You want it running as good as you can before you head out on the roads much.
Right behind that - clean the headlight lenses. Or replace with factory (OEM) or factory style.
Put new bulbs in the headlights.
DO all that so that you can see at night.
THEN ... THEN ... Then you start doing the sexy sick neat cool schtuffs
But keep it safe!
We can always replace a damaged truck. You, OTOH, are probably a bit harder to replace ...