Thanks to everyone for the info and the welcome to rec.woodworking!
The ASCII diagrams were especially useful. With this post I was mainly
concerned with the side walls. I will likely use (purchased) clear
spans of 2"x4" for the roof in the interest of safety. Although, if I
can figure out a solidly engineered truss system using the same
construction cast offs I might go in that direction.

Mending plates will cost extra so, I'll probably pass on using those
for the walls.

My point in all this IS to be frugal. Currently, we have one half of a
garage filled with lawn & garden equipment and I would like to reclaim
this area for a workbench and to park my car. We have the "benefit" of
living in a development with several houses under construction.
Currently, two have been sheetrocked, two have just had foundations
completed with lumber delivered. Another has just been sheathed.

I have a qty of leftover exterior grade screws I will use as well as
some nails.

I also have a reliable source of wood pallets through my wife's
employer.
I will be examining these next for the 2"x4" used. I am aware that the
quality of wood varies widely in pallets and may not be worth using
(or even safe).

For those interested in tracking my progress I will post a page soon.

So far I have scavenged the following cast off materials:

2 pkgs of cast off roof shingles (black)
30' of 2x4 mostly in 8' and 10-12' sections scavengend from a cast off
truss.
8 concrete blocks. These have been already cut in a "half/L" shape
which I will use as perimeter "piers".
1 partial roll of vinyl flooring.

What I plan to buy:
pressure treated lumber for the floor framing
Some exterior paint (keeping an eye on the "oops" cart at HD &
Lowe's). Alternate plan: I have seen some scrap vinyl siding but, will
keep an eye out for the right color.

Rob A.(who's usually on alt.food.barbecue)