A razor plane, or a nice standard plane and some skill with a sharpening stone is essential in my opinion. I also like to have a 2x4 thats about 2 feet long (you are building a rather large plane, you may want more length) for a fine finish sanding along the whole leading edge, or anything requiring a very consistent long sand. I also keep one about a foot long with sanding screens for less precision.

Clamps are essential as well. All sizes, shapes, everything from a bag of clothes pins, to a large spanning clamp. Throw some c-clamps in there as well, I have a few 2 inch ones that are in use quite often.

Thats all I can think of off the top of my head, I'm sure others will jump in this.

eniac

edit:
I forgot to mention some angle-aluminum. I cut eight or ten chunks about 2 inches long and use them to reference a square with the building board. Handy when building a wing and wanting to know all the ribs are square, or at least pretty close, its probably not perfect. I think I have 3/4x3/4 inch, but I'd have to check. Its just something you can strap the clamp to, and weight it to the building board so its not too bad.