I have a 98 Expedition. They are great trucks.
IMO its a good thing you went for the Navi. The 5.4 4v is alot of fun and those trucks come loaded.
The air
suspension thing is a hit or miss, sometimes they go while some people don't have any problems.
Since it's not a "new" truck get ready for some of these things to go:
-rear subwoofer may seperate from the magnet and start rattling, just replace with a matching unit from another company.
-bottoms of doors may start rusting(most likely)
-IMPORTANT!!! Make sure the Cruise Control recall ahs been done. Call the local Ford Stealer and inquire.
-Get Bilstein Shocks(You can only get the front ones for the Air
Suspension models but it will still be worth it)
Good idea that you're gonna do the plugs(Have fun with #8 BTW), you may want to change the COP boots while you're at it. They are cheap at Autozone. Use plenty o'dialectic grease too.
They are really good trucks, you should be happy with it. The 2nd Gen. Navi's/Expy's are awesome also. But i like to have a solid rear axle for towing haha.
Good luck with it and post up some pics!