Thicker is better.   How thick is dependent on what tools you will use to
make holes in it ( dog holes, mortises, etc.) and what you can get your
hands on.  There is also the issue of how you will mount any vises to the
top.  Some vises require a bit more meat to hang from.  I think the Veritas
twin screw has some requirements on the top thickness and the skirt
thickness... Study the directions before you buy anything.

 What kind of wood?  I have seen ... Hard and soft Maple, Beech, White Oak,
Iroko, and some form of Rosewood.  I have heard about White fir, Yellow
Pine, cherry, and ash.  The what kind is best handled as whatever you can
get your hands on that meets your budget, is straight and true, flat sawn,
properly dried, and as hard as you can get.

The base can be from construction lumber, but dry it out thoroughly first.
The more massive the better for the horizontal members.

Frank Filippone, working on Bench #5
red735i@e...