So first order of business is the bed.
I can cut and put screws into wood, I can even use glue and come up with ways to use hinges, but constructing a mattress was something of an enigma for me...
I called some places and got outrageous prices for custom made foam mattresses (and I'd need two if I wanted it to fold!)... and then we went to IKEA. There we saw a nice 4 foot wide futon with a two piece mattress. Fortunately IKEA sells the mattress and futon separately, the mattress costing about $90. The two halves were not the same width, but it was close enough to perfect - at exactly 6 x 4 feet together.
Now that I had a mattress I could build the bed - which was made out of two 24" sheets of 3/4" edge-glued pine, a few 6" pieces of the sides, and a few hinges creatively installed. For the base I used two IKEA mini coffee tables, and used the bottom shelves they came with for front cabinet doors - now I had decent storage space under the bed too. Incidentally, the rear center of the assembly is over the wheel well, so I didn't use the legs there - instead I bolted a piece of 2x4 through the well hole that used to secure the seatbelt.
There were a couple of minor tweaks to the bed (the legs now swing open when you open the bed, and close when you close it), but for the most part it worked out well. I also added a little wooden lip to hold the two wood panels aligned when deployed.