That's a very nice truck, I wouldn't get in a hurry to start modifying it. Like the other guys said, a clean stock OBS is getting rare these days. Too many of them got hacked up by teenagers.
If I were you I'd just do some small stuff. LED lights all around, polish or replace headlight housings, figure something out for the radio, (depends on how much you want to spend) and track mpg to make sure it's running right. Other than that keep it clean and keep it running, and keep good tires on it. OEM parts are 100% worth it on these trucks, the Doorman stuff is crap. Buy Delphi or AC Delco whenever you can, and always ask here for better alternatives if you can't find it.
What's the mileage?
Some major issues you are bound to see at some point are:
-Fuel pump
-Injectors. Be sure to get a genuine Delphi spider unit. Be very careful because the bolts to access this are very brittle and weak. I broke two, even being careful with a tq wrench.
-Intake gasket
-Timing chain, again these bolts are old and brittle. Get new bolts too if you decide to do this yourself. If you break these you have to open up the whole engine and replace the cam. Ask me how I know.
-Trans rebuild around 160-200k ish, it really depends on how it's been driven through
-A/C repairs if you let it sit too long
-Various sensors and electrical bits
You don't need to rush out and do these now, just make sure you keep a rainy day fund for the truck, you'll need it at some point.
Some of the mods that I have done that are worth it are upgrading to the 2500/3500 front calipers and the 9.5" rear end (stronger and better brakes). I have LED lights all around on the exterior, those have been great. I went through and did a whole audio system, all Alpine everything with two amps, one for the 12" sub and one for all the speakers, and 16g wire running to everything. My head unit has navigation, Bluetooth and an AUX cord in the back that runs to an iPod in the glovebox, along with a bunch of other stuff. A winch is very useful, Rough Country makes a hidden winch mount. I have it and their winch on my truck and I love it. I also have newer style towing mirrors. The visibility is excellent but the fit is just ok and they look a little goofy IMO.
I went down the lift/tires/gears rabbit hole and while I enjoy it, it's completely impractical. A set of tires is $2000 now and I get 10mpg. Resist the urge to lift your truck unless you are ready to spend this kind of money to keep it up. It's also just a pain to get stuff in and out of the bed. If you must go bigger, stick with a skinny 33. 285/76r16 fits on a stock truck with stock wheels just fine, they are easy to get and have all the usual brands/models to choose from. Once you go 37+ then your options get limited and expensive.
If you want a lumber rack and a camper shell you can do that, you just need to find a rack made to go with a shell, they usually stick farther out the side and go around the shell. I would look for a used one on Facebook or Craigslist or whatever, get one that is fully welded. I had a bolt-together one and it rattled a lot and just looked cheap.
It also looks like you need a bedliner. You might consider adding more tie downs to the floor or sides of the bed, I often find myself wishing for those.
