Definitely the least expensive route, not to mention the one that removes the questions and headaches, would be to get a ready-built 4x4. Making a 4WD out of a 2WD is best left to those with serious budgets making serious project vehicles, or someone with a very well-equipped fabrication shop. Like the old saying goes, "if you have to ask", then you probably shouldn't be attempting your own conversion. Your current truck ought to bring enough to get yourself a decent 4x4 to work with, especially since you have some side money to put into it up front. A stock or mildly modded older 4Runner can take you many places.