You can have tons of fun with stock 235/75R15 tires, but you can upgrade to 30" tires; they are only about 1/2 an inch taller than stock, but you should be fine. BF Goodrich AT would be a good choice for a rig that sees double duty, as a daily driver/off-roader.
Suspension, depends upon the type of wheeling that you do. If you do mild wheeling that a stock vehicle can handle there is no need to upgrade, however, if you really like to push your ride, you need a lift kit. I DO NOT suggest doing a mild lift kit (1-2.5"), because those type of lifts, increase CV angles, and cause accelerated wear of the suspension components!!. If you want a lift kit the only suggestion is the BDS 5" kit, Superlift 6"; but not the Trailmaster kit, it's not all that great, but BDS is the one I suggest!!.
You can go ahead and add clearance, by replacing the fenders and bedsides with fiberglass components, which will add width and increase clearance for larger sized tires, without affecting suspension geometry, and you can fit 31"-32" tires on a stock rig. Search for Hannemann fiberglass, they have stuff for the s-series. Or you could keep the steel body panels and add a 3" body lift, I don't like them, but it's entirely up to you!. Lastly, you can keep the vehicle stock, no fiberglass or suspension lift, and you could bite the bullet and trim the fenders, then you could fit up to 35" tires on a low center of gravity!!.
As for the air intake, I'd suggest K&N they have the second generation intake kit for the 4.3 it comes with a heat shield, if I remember correctly. There are several, you just need to choose which one YOU want or can afford. Some other kits are the following: SB filters, Green filters, Airaid, AEM, K&N, and several more. You can get one at your local auto parts store chain; one of the larger ones, such as Kragen, Schucks, O'reilly, Pep Boys, Autozone, etc.
Those are my suggestions, ultimately the end decision is up to you because it's your money and your vehicle!!. Hope that helps you out a bit, there are tons of websites that are dedicated to the modifying the s-series!!. Late.