You're getting good advice here. Look at wear parts (brakes, belts, hoses, filters, etc.) and fluids first, as those are easy and usually relatively inexpensive compared to major mechanical rebuilds or replacements. Then start thinking about some of the parts which will obsolesce in terms of function like window switches, relays, actuators, etc. If you haven't looked it over, consider reviewing the thread on refreshing a first gen titan. It may already be a sticky, but it's easy to find with the search function. Plenty of good advice there.
Beyond that, your truck is going to tell you a lot about what it needs. Climb under, around, and in the truck, getting your head and eyes on everything you can. If a bushing (sway bar, steering rack, etc.) is all cracked and dried, might think about sourcing a replacement. Same with mounts (motor, trans, transfer case, leaf spring, etc.). If you have dirt or grime, clean it off. If it returns quickly, look for fluid leaks or grease being thrown there. Address those. You know your truck better than anyone (or at least you should) and it shouldn't be too hard to see/hear/feel all the possible things. Take your time and make a list and evaluate the best bang for the buck if you can't do it all at once.
As far as brakes, have your rotors mic'd before you replace them. My wife's 2004 Expedition with 193k+ on it is still on the factory rotors, having had them turned once. We bought the truck new in 2004, so I know all the history on it. I suspect they will need replacement soon, as they're starting to pulse again, and I'm doubtful they can be turned a second time. I'll check, though, as those parts don't really wear out until they're too thin to be turned again. Might save you substantial money on things like that.
