Depends on 2 things:
1) What are your goals?
2) How much money do you have?
Is your goal to go to have a full on off road race the truck? Or is it you just want a street queen with that "prerunner" look, or something in the middle?
Once you have a goal for what the truck will be used for then comes the money question. The smaller the truck (generally) the cheaper the build.
I see a lot of people start off with an old single cab 2wd ford ranger or toyota. These old small trucks can be purchased very cheep, so the cost of entry is much lower. Plus there are a lot more of them around so its easier to get used parts for them.
How ever much money you think your going to spend get ready to be very wrong. I think your best bet would be to find some one selling there prerunner. Because for what you are going to spend to build it you could spend about half and buy some one else s. I have spent probably over $60k on my truck (to include the cost of the truck) and I would probably be super lucky if I could sale her for $20k (I will never sale her).
Good luck on what ever you decide. Its a lot of fun to be able to jump a full size truck.