The coils on the Discovery take a little work to get to. You do need to remove the plenum. I'd recommend replacement of the gasket if you do. The bolts are long and skinny- be mindful of the torque spec and the thread lock requirement. The coils are pretty rugged- I wouldn't worry about them unless you are sure have a problem. I'd be more concerned about what's downstream. A good set of aftermarket wires will help your truck's performance, even over a stock grade replacement set. And there are alot of differing thoughts on spark plugs- and plenty of debate and recommendations can be found here.
While you are this far, you should probably do the valve cover gaskets as they are probably leaking pretty well. The stock one's harden to a brittle plastic state. Be careful removing the SAI tubes. The adapters that go between the tubes and the head are brass and the very narrow flats round off easily. Uou have to get a wrench on the flats and one on the big nuts on the tube to remove. I would liberally soak these fittings several times over at least a full day before trying to remove. If not, you may twist one. These tubes have (or at least at one point had) a fiberglass insulation on them. If it's damaged or missing, re-wrap them with skinny header wrap tape. I use a copper wire to wrap over to retain it. Without it, you'll cook the wires.
You'll still have to pull the plenum for the wires, but it's an hour job off and back on. It just looks like alot when you first glance at it. It's not. While you're taking it apart, be aware of the IAC motor (silver cylinder with hoses going from air inlet into back of manifold) and beneath it, mounted to the plenum is the control for the SAI system. It injects air into your exhaust at startup. These are items you may be dealing with in the future and now you know where they are and what they do. Take this time to make sure the little rubber elbows on those lines are intact and the lines aren't rubbing through. The vac lines connect to two little cans mounted to tubes running to each exhaust manifold. on the other end there's a stainless tube that runs back and over the coils. it tees over to a miniature shop vac mounted on the right side of the firewall. It has a filter in it- replace or at least clean it. The tube that runs from the output up to that stainless pipe can deteriorate with age. It is squeeze-to-release on the pump. It's replaceable with heater hose. You may just want to go ahead and do that now. Also, you're going to have to remove a little gizmo from the firewall to get at the SAI pump. It's a roll-over switch. Remember to reset it.
Back to the coils- be aware there has been some anecdotal discussion of problems arising when the coil packs are missing mounting bolts. There may be a reason such as proper grounding. I guess there's no reason not to have all the mounting bolts present, but four fasteners on a device the size of a pack of cigarettes seems like overkill too. I can see if someone dropped one how they could think "aw, F-it". I've seen it more times than not.
I would be very cautious about cooling system issues. I would definitely replace the thermostat and would suggest the Motorad 180 degree unit. Myself, I would replace every hose, but at least be aware of hose condition and replace anything that looked even remotely questionable. The rover uses some hard plastic cooling lines that can and will crack. One goes from the top left of the radiator tank back to the fill bottle. The other goes over to the throttle body pre-heater. That little cover on the bottom of the throttle body is also notorious for leaking. While there, you might as well replace the cap. Be aware that the nipple on that recirc hose on the top of the radiator tank gets brittle and will snap off easily. I have repaired several using an 1/8" NPT to barb fitting. It's a little tricky cutting the threads at that angle, but not all that hard. Use lots of thread sealer. The fill bottle on the right inner fender can also get brittle. The top radiator hose tee, with the bleed fitting will also crack with age. If you've ever been under the hood of a late 90's or early 2000's BMW, you will recognize alot of these pieces.
Take a look underneath and make sure the heat shielding is in place. It sometimes gets discarded by people who don't know better- or don't care, when working on these trucks. The cats are tucked up in there and the exhaust is tight. If the starter heat shield is missing, you'll have a problem in the near future.
There is alot of discussion about oiling with these trucks. Many recommend using Rotella 15W-40, which is a diesel truck oil. It is rated for gasoline engine use as well. I'd suggest the T6 product. The heavier weight will help make up for some internal wear common in the D2's engine. And it's fantastic oil. There's also a Purolator filter available that fits this engine and offers about 4 times the filtration and a higher pressure bypass. Readily available at Advance, although probably a next-day item.
Back to cooling, check to make sure your auxiliary fan is working. It will come momentarily when you first start the car cold and will also turn on when the AC is turned on. Very common failure item. You can reach through the grille and try and spin it with a ling screwdriver. I use an aftermarket from Summit Racing that works great and costs a few hundred less than an new one from Rover. Another item to watch is the fan clutch. Your 03 may or may not have the one-piece fan and clutch that all '04's have and they're expensive. You can use a non AC Chev S-10/Blazer unit in its place and they're dirt cheap. It will need some fan shroud adjustment for clearance- NBD. But they really pull the air. Great mod for warmer climates.
I'd definitely recommend a trans service. This is a PITA. Drop the pan, replace the filter- there are two different ones- make certain you get the right one. Interestingly, RockAuto sent me the wrong one. Advance had the correct one in stock in a little store in a tiny rural town. Go figure. Anyway, to refill, you have to fill it while running from underneath, drive it and top it off. It will probably take three fills to get clean fluid out of it. I'd recommend a good synthetic. You'll probably have at least a hundred bucks in fluid, but its worth every penny. While you're under there, service the transfer case and differentials. Use good stuff- Redline is my first choice but Royal Purple works well too. WalMart generic is better than not doing it. You can go a little heavier on weight and damp some gear noise in the TC.
At 120K there's alot that that's probably marginal and common to replace on any truck this age and mileage. Steering components, shocks, I'd also pay close attention to the brake lines. You'll probably also get a visit from the 3 Amigos. It's three lights that may intermittently appear on the dash. This is a switch in the ABS controller that costs $65 and takes 10 minutes to replace so don't sweat it.
You will probably experience some door lock malfunctions too. They are also BMW items and suffer from the same problems. Don't bother buying used one's. They aren't hard to swap but they're also not cheap. Like all the BMW units, they are riveted together and to me they don't look like they're worth the effort to attempt to repair. I've had to drill out more than a few of the Torx fasteners that hold them in. They're easily replaceable with a button head with a socket hex.
That's what comes to mind for things to tackle first. I'm sure I missed something.
Good luck!