I would stear away from the diesel powered Liberty. I have heard nothing but bad things about them. The 2.8L diesel has no power and worst gas mileage than the gas powered version. Also the Liberty really isn't a great platform for off roading. It can be done and I know people that do off road them, but there are easier and cheaper alturnatives.
I know a lot of people that have built vehicles like you are looking for. Of coure I have to suggest Jeeps and they all make good starting points for X vehicles. Wrangler/CJ if you want the most options but smaller size. Cherokee if you want more room inside, and a Grand Cherokee (ZJ or WJ) if you want a bigger engine option (V8) a little more room on the inside compared to the Cherokee and more comfort.
As others have suggested, finding diesel can be a pain at times in some areas of the Nation. But my biggest suggestion for a X vehicle is choice a platform and engine that isn't difficult to find parts for all around the world. Nothing would be worst than being broke down in some remote part of the nation and having to wait a week for parts to be special ordered. Because if you really do that, you will break down, and you will need to fix something along the way.