Just checked out this thread and I figured I'd add to the encouragement pile. I learned to work on cars early on as well, and am always glad to see someone so young have an interest in a mechanical trade. Building your first vehicle will not only be a great learning experience from a mechanical standpoint, but from a life lesson standpoint as well as you will be more proud of the truck and treat it right since you have put the sweat equity into it. You will always be able to point at it with pride and say "I built that".
From a project management standpoint, and don't take this as a bad thing, you need to curb your enthusiasm and stop throwing your time in various directions at once. Make a checklist of everything the truck needs to run. Start with the electrical, work towards the fuel, then the little stuff. Then follow it religiously so you don't lose focus. It's easy to want to play with little things, but you need to practice focusing on the necessary so you can get to the fun stuff, trust me, I speak from experience in this as I always wasted time fiddling with unimportant distractions. Once you get the engine sorted, then you can start looking for the rest of the missing driveline. There are lots of generous people on here, and once you get closer to needing those parts, opportunities may crop up.
Also, don't chase rabbits on CL. Decide what you want, then go find it. Otherwise you'll never get something. I would recommend finding a transmission/transfer case that are already put together, and preferably that came from a running truck. NEVER buy a used transmission that has no oil in it. You have no idea what the inside is like because you can't smell/see the fluid condition. Also, getting a transmission from a diesel will save you the trouble of digging up the proper torque converter.
Final thoughts, let me give you another kudos for your online communications skills. It's rare to find someone your age who can hold a coherent, technical conversation without 'internet grammar'. Trust me, that gains you HUGE respect and people take you seriously because of it. Always remember that.
Hang in there, and trust me, there are many people here who are looking forward to the day you post up pics of you driving that beast under its own power for the first time.
Later,
Joe