D,
I just "finished" my build. Obviously the amount of effort, time, and money will vary with build complexity and individual tolerances for perfection. All told, I spent approx 1000 hrs over the last year planning and building... had a great time and learned a ton.
What I learned (in addition to the "nothing is flat, nothing is straight, nothing is perpendicular, nothing is parallel" wisdom offered above)
1. If it is worth doing once, it'll probably be worth doing it again.
2. Define what "done" means to you... before you become obsessed... or go insane.
3. Measure & clearly mark all reference points (e.g. centerline floor & roof, perpendiculars to floor on wall frames, front & rear reference points for lengthwise measurements) before you put on that first piece of sound mat or insulation.
4. If you are at all worried about loading or weight distribution, weigh your van before you start... mine worked out fine but wish I had been more deliberate... I was more lucky than good.
5. Be extra nice to your spouse... they will be putting up with a lot of crazy behavior of the next several months/years.
6. Set aside approx 20% of your built budget (not including van purchase) for miscellaneous necessities... those extra little packets of screws, fittings, and tubes of adhesive really add up over the course of a build.
7. I accomplished 100% of things I put my mind to and 0% of the things I didn't... I only started a job when I was mentally ready for it (i.e. thru research, asking questions, actually reading the instruction manual)
8. Templates are your friend. I wish I had made templates or profile sheets of the floor/lower wall, middle wall, and upper wall/ceiling when my van was empty... would have saved time and improved panel fit.
9. The individuals here on the Promaster Forum are a tremendous resource... experienced and not judgmental.
10. I haven't really "finished" my build... I'm simply testing it and saving my pennies for the next round of improvements.
Best,
Phranc