Tom, I have built a few of them and as others have said they make a nice model. The underframe fit problems Paul mentioned was a flaw on several batches that was later corrected, but is easily corrected by any resonably skilled builder.
With the kit experiance you have listed you will have no problems. They are very similar to Proto, Tichy or Intermountain in tems of detail and dificulty.
The reason the instructions are not real specific about the underbody is because there where lots of variations on the prototype. The same floor plan car might have a different arrangement of equipment depending on built date, rebuilds, preferences of the leasing road, etc. The underframe parts in the kits are actually somewhat generic and do not represent every possibility.
There are some greeat deals out there right now as Branchline just offered some super deals to dealers.
Sheldon