I will second the notion on a 4runner as it will take a good bit of money to make the XJ a sturdy enough vehicle to get to Toyota level. It will be a choice you will not regret. If for some reason you are inclined to pursue a less expensive build that will be along the same lines (except with more room) you can go with a 2nd or even 3rd Gen Montero. The aftermarket isn't as available as a 4 Runner but IMHO you don't need all that much for one. Just be sure you have a good timing belt and you are good to go. Initial investment isn't much at all and you can easily make a very capable overland rig for a fraction of the cost of a Toyota and a good bit less than a Heep. I mean Jeep. No offense.
See my build thread if you are interested although I know that when most people see it, they have no idea what it is.
If this isn't an option for you, please go with the Toyota, but be sure to get one as stock as possible. Competent reasoning will enlighten you as to why. That being said, if you can get a very good deal on an XJ and are willing to put better axles, joints, driveshafts in it then that would be a good choice. The final choice is up to you and you seem like a bright kid. I am sure you will make the best decision, but if you do not it will only be a lesson learned. You will have many ahead of you.
If you do have the coin and can afford to drive it, I'd go with a Land Cruiser over any of the other choices. I know that not everyone (including myself) could afford a vehicle near 10 grand at 16, but if you can..... I say do it. The reliability will save you money in repairs that could be spent on gas that a less reliable engine will save you. Most Land Cruisers have been well kept since the majority of the people that have owned them tend to spend the money on upkeep since they are so expensive to begin with.
In conclusion- If you want and can honestly afford a LC then by all means get one. If not, the 4 Runner is a great option. Let us not forgot the Montero if you can find a good example to purchase. Lastly, the jeep could be a good first rig as parts are found in every crevice of every parts store in the USA. Just remember to beef up the drive train and the body as a Cherokee is a built as a uni-body and not an on-frame build. (google the pros of using an on-frame build vs uni-body for overland use)
HTH