A great question Daniel!
The amount of preventive maintenance required for the XLR (and most modern cars) is pretty low, compared to the vehicles we grew up with.
Remember when a car with 100k mi was considered to have one wheel in the auto graveyard? Thankfully, those days are long gone.
Depending if you're the first owner, or have the vehicle's service records available, (you can get them from your dealer if you furnish the VIN) you should have a relatively good idea as to what needs to be done. If in doubt, have the vehicle inspected and replace whatever fluids your mechanic recommends replacing.
I'm a firm believer in "If it works, leave it alone." but I usually temper that with "Trust, but verify." An '05 could be up to seven years old if it were an '04 build, so you're smart to keep the condition of the fluids monitored. 20k mi is pretty low too, so mechanically, your car is barely broken in!
For long-term storage, add fuel stabilizer and drive the car enough to get it fully mixed thoughout the fuel system. A Battery Tender or Minder is another must-have. There's a long-term storage post here on the forum you can check out. As for fluids, I had my cooling system flushed after five years, followed by the differential with friction modifier added. This might be a good time to really get under the hood (and car) to inspect fittings and gaskets for leaks. Lube up the rubber seals (especially around the roof.) There are lots of good products available and everyone has their favorite. GM recommends Krytox (so do a lot of Navy divers) to keep them pliable.
Also check to ensure the rear/left compartment drain is not clogged. Flood that area and you could pay enough for more than two new sets of tires.
To make a long post short, I hope this answers your question. Peace of mind --priceless. For everything else, there's Mastercard!