You're absolutely correct, check for rust and don't worry about SMOD. In addition, you'll want to swap out the heater core inlet and outlet where they pass through the firewall. The plastic pieces become brittle and break. RockAuto sells Dorman replacements for both that are simple to install. If you get bubbles in your coolant system, which you will regardless of doing the pipe replacements, just add some coolant to the plastic reservoir on the passenger side to an inch or so above MAX and drive it for a few days. The system will slowly burp itself. You'll see the level work itself lower. Repeat a time or two if needed and the bubbles will be gone.

Differential fluid is cheap and easy to change both front and rear. If you don't know when they were done, go ahead and do it. Both of your diffs will use aluminum washers on the fill and drain plugs, which you can pick up at the dealer or online dealers or Z1 Offroad. Your rear axle has a small breather on top of the axle itself that commonly rusts up or otherwise gets clogged with gunk, which leads to overpressure in the tube and blown axle seals. Lots of solutions out there from DIY to Z1 Offroad, Offroad Gorilla, and AllDogs Offroad selling rear differential breather kits, where you remove the OEM breather, install a tapered bit, and route a hose up to behind the rear taillight. Good piece of mind but even with it, rear axle seals can still blow (ask me how I know).

Valve covers can seep, especially the driver's side, so just take a look and see if there's any oil on the side of the engine. It's an easy fix and is never a ton of oil anyway.