If you're going to be doing a lot of offroading and expeditions I would get some 15" wheels for your offroad tires. With the money you will save on the tires you will easily be able to pay for the wheels and you end up with tons more tire choices and an easier time getting a replacement tire out in the sticks, if you need one. You will also get a better ride when offroad due to the extra sidewall.
As far as tires go, the Geolander is a good A/T for mild offroading, but it's not as sturdy and won't last as long as the BFG A/T. I would say the Bridgestone Revo would be the next in line to the BFG with more onroad performance, but not as sturdy offroad. If you go with an M/T, the Bridgestone DUeler M/T and the BFG M/T seem to be the best compromise between offroad and onroad. Most other M/T's are too agressive for street use and either won't last long or will vibrate badly on the the road.
If you put a bullbar and winch on you will definately want to upgrade the torsion bars. Get some Bilstien shocks too while you're at it.
Skids are a good idea. The ones Nissan gives you aren't much more than decoration. The engine oil pan has no cover at all, nor does the automatic transmission pan, if you have the auto. If you're not rock crawling and you're careful, you can probably get by, but you should definately keep them on your list.
Rock sliders are a good idea too! Calmini and Schrockworks make nice ones.
Good for you for taking the course at Offroad Academy! A few of us have spent time with them and their setup is most excellent. Nice guys too!