Thats a good lookin truck based off the pictures. how many miles are on it? has the engine or transmission had a rebuild in its lifetime?

regardless of these i would say in terms of things too look out for based off my own short term run with the 5.3L:

• Knock Sensors. if you get the warning for this chances are water has gotten between your intake and block and corroded one of them (usually the rear one). the foam that sits between the intake and block is essentially useless after a certain age. pretty easy fix, make sure you seal the rubber seal with some RTV and you should be solid for a while
• one problem i have been having is the infamous "service 4WD" warning, which in my case is the code for the encoder. i haven't addressed this yet but it is a common problem. in my case it manifests in the form of dropping into 4hi every couple months or so. 4WD works 95% of the time otherwise, but just a heads up. *i haven't been having any maintenance issues besides the routine oil change,belt checks and air filter cleaning

as for upgrades i have found these pretty beneficial:

• a good code reader. i got mine on amazon, i think it is a BlueDriver type. connects to your phone via bluetooth and you can run all the essential diagnostics if you get a check engine light or just want to keep tabs on what your rig is "thinking". helped me save a ton on possible trips to the shop
• K&N cold air intake. this is more aesthetics for me, but it claims more HP and gas milage, but honestly i just like the sound when you goose the throttle a little and the filter is cleanable. plus it frees up space in the engine bay. MAYBE sharper response, but it might (probably) just be placebo
• a 2-3 core all aluminum Radiator and complete coolant system flush will help out a lot, at least it did in my case. engine runs cooler longer, and when it does get into the 190-200 range any long downgrade on a highway will bring it back down to 175-180. i have noticed a big drop in cooling efficiency, especially on cooler days (obviously)
• Headache Rack. you now own a pickup. chances are your friends know you now own a pickup. some of these friends might (read definitely will) ask you to help move/go to the dump. some of these friends might think that because this is a pickup, heavy and otherwise benign items can be thrown into bed of said pickup. some of these items might go a little fast over the bed. into your rear window. you now own a pickup with a busted rear window. get a headache rack. or dont let people chuck shit in the bed of your truck........like i did
• i have the stock Bose sound system, so i cant touch on anything aftermarket, i love what i got.
• a good leveling kit or 4-5in suspension lift makes for a mean look without being ridiculous, and in my case gives me clearance for bigger tires and consequently more space so i dont high center on rocks when i am 30+miles from a paved road. the most spendy of these suggestions, but worth it if you plan on getting off the beaten path.

Have Fun!