As others have posted above, there is no "golden rule" for the first step towards building an expo rig. It all depends on your intended use, your budget, and the vehicle's current condition. How many miles does it have? How much life is left in the tires? Does everything work?
Also, if you don't have maintenance records for when the last service was performed, you should consider a full fluid change:
oil/filter
AT fluid
fr/rr diff fluid
t-case fluid
brake fluid flush
check coolant condition
Depending on mileage and/or service records, consider replacing the water pump and timing belt. Reliability is critical.
That being said, when I purchased my '05 Sequoia last year, I knew the direction I was going to take with it. After doing all of the major maintenance above, I fixed everything that was not functioning properly. I hardwired the rear cargo area's 12v power socket for running my fridge, which was the most important mod for expedition travel. Gotta keep the food cold!
If you'll be driving on mild dirt roads for extended periods of time, one of the best things to get is a portable 12v air compressor such as
this $60 one, so that you can reduce air pressure for a more comfortable ride and for better traction, then reinflate the tires before reaching pavement at the end of your trip. Recovery gear (shackles, tow strap) is also important to have.
Unless you're driving on very rough terrain, you probably won't need armor.