The 20" depth probably wont make you happy if you're going to keep anything on the work surface (like storage bins, tool organizers, etc.). I think an equally important dimension is height. My workbench is a bit over 40" high - perfect for me to work at while standing (I'm 6'1").

Ken the Carpenter's advice above is sage... You can build a very sturdy workbench out of wood and simple fasteners. Mine is similar to what he described, and it probably cost me $50-60 in materials. I did it myself and I most assuredly am NOT a skilled woodworker!

Go ahead and DIY it... You're a good mechanic, this'll be a piece of cake.