I'm in about the same place you are, Eric, so I know what you're saying. This forum is a bit overwhelming in that there are so many awesome builds, design ideas, techniques, and tricks that pretty soon I find my head spinning and I don't know which way to go as a first step.
Finding and settling on a design was the hard part for me. Just about the time I thought I'd found the design I wanted to build, I'd spot another design someone was building and off I'd go again. I think I've downloaded every design in the both libraries here and I've gone over them all with my fine-toothed comb so often, half of the teeth are now missing.
My problem is that I'm not only easily distracted by shiny objects, I just simply want them all.
For my very first build, I finally decided what I wanted the trailer to do, then looked for a profile that would fit. When I found what I thought was the "right" one, I tweaked it a bit here and there to fit my frame, did some mental engineering, and finally made the command decision that this was the one.
The suggestions given by everyone so far are the suggestions I finally took - it's great to see them all in one place. I'd had Google SketchUp for a while and was learning to use it for my woodworking ideas, so it was a fairly easy thing to use it in this situation. If you've never used it before, it can look pretty intimidating at first, but the learning curve is so gentle that even I was making simple, basic, easy to read drawings within about an hour. They have an excellent series of tutorial videos that are really a must for someone just starting out. And the best part, as already mentioned, is that it's free.
Now that I have my design, have it drawn out, and have put it through its virtual paces in 3D, I'm ready to go. I haven't built a thing so far - I've just basically rebuilt the frame I plan to use for my first build. I've assembled a lot of parts and accessories I plan to use and I'm making a list of what I need to do and when I need to do it. In that regard, this forum is AWESOME. Every time I think I have a question, I find the answer in someone else's build thread.
Steve Frederick's guide has been mentioned several times, and I plan on getting it. I got lost over at his website for a couple of days, and found my way out of it a drooling basket case.

His work really has to be seen to be appreciated.
I guess my point is that I didn't think I needed to buy any plans when this forum is such a fantastic resource - your mileage may vary. Just looking through the design library will show you that about 99.9% of the engineering has been done for you. True, we all want to make our trailers unique, but there are probably a gazillion ways to do that without changing anything structural in that design.
What I'm doing is following the path already made by others here for my first build. There's no limit on the number of campers I can build or own in my lifetime. If I hit the nail right on the head on my first attempt, which I doubt will happen, I'll sell it or tear it down and try again. Either way, I don't figure I'm wasting either my time or my money to get experience. I was told years ago that experience is more valuable than the time or money you put into getting it, and I still believe that. By following someone else's lead I'll learn how to do it, what to watch out for, how tab A goes into slot B, and how to basically plan ahead for my next build. I can branch out on my own and build something truly unique later on down the road. Right now I think I need the training wheels. Mountain biking is still off in the future, and I'm in no hurry.
Dusty