The rust is certainly a concern. I don't have a lot of experience with rust in my area. I do have some mechanical experience, and from a mechanical standpoint, I offer the following:
Check for excessive blowby... remove the oil filler cap with it running. If it blows the cap off, there's an issue.
Check the exhaust manifold. These trucks are very sensitive to overheating, which isn't common without poor maintenace or mechanical malfunction, but it can easily cause a broken piston ring, which can take a while to rear it's head... look around the exhaust manifold for signs of oil residue leaking between the exhaust manifold and the block. It's appearance indicates oil being burned by that piston, which is a serious mechanical issue. It's also a good idea to look for exhaust leaks along the manifold where it may be cracked. They will appear as dark black/charcoal-looking lines along a rusty metal manifold.
Look for oil/fluid leaks. Obvious, in my mind, but I'm surprised how many people just don't think of it. Crawl around under the truck. On 4x4s, a front pinion leaking is common, but it can also indicate a vibrating driveshaft. Transmission leaks are common along the pan, but check for line leaks, as well, if it's an auto trans. There shouldn't be any red transmission fluid leaking from the engine area, or there's a possible issue with a seal that's very expensive to repair. Look at the back side of the radiator by the fan for antifreeze seepage and bent areas that may leak. On automatic transmission trucks, look for antifreeze up by the back of the engine on the driver side. There are rubber lines that are prone to leak from age on a heat exchanger bolted behind the block, above the transmission casing.
Engine oil leaks can be an issue, as well, and if it's leaking excessively from multiple points, it may indicate excessive blowby internally. Always ask for maintenance records, if they have them. A reputable shop that works on trucks can perhaps give insight to a trucks history, as well.
I've had a lot of bad experiences from aftermarket filters.... if they're not up to par, they can cause a great deal of internal engine damage. For air filters, K&N and most other oil-bath filters are on my NOPE list. I've got a box full of worn and broken piston rings that have come out from behind K&N products. If in doubt, pull the intake tube and look inside towards the turbo. If there's oil or dust there, I'd be leery of it.
U-joint issues are common upwards of 100k (or less if that particular one didn't get a full shot of grease at the factory before install). Push/pull on the front/rear driveshafts, looking for play, and look at the front U-joints in the steering knuckles, for wear and slack, if it's a 4x4.
Ball joints are another weak link to look for, so try shaking a tire in and out, looking for slack. This only shows up in the worst of cases, so to be sure, it has to be lifted off the ground and tested.
Look under the dash for any aftermarket electronics installed, and how the wire loom looks there. I've seen some really clean trucks that have turned into nightmares after the wirelooms have been mutilated from radio, programmer, and custom electronic installs by "less than professional...."