Hey, dguy, you won't find much furniture anywhere, except from the Amish in PA, that is solid wood any more. Even Bassett and Thomasville use pressboard, etc.
Hardware for the file cabinets, etc., is available from Woodworker's Supply - hubby got all of our office cabinet hardware from there when he built out my (former) boss' office. He built 6 2-drawer file cabinets about 5 years ago that are still going strong.
As far as a suggestion for materials for your cabinet, we've had huge luck using 3/4" double-sided melamine, faced/edged with solid material. My kitchen cabinets, china cabinets and buffets, pantry cabinets, computer desks, are all built in that fashion. I'm attaching a couple of pictures from our laser shop showing the desks that he built, just to give you a starting place to think about.
These are small, one-person desks with no drawers, as I have a couple of separate filing cabinets so didn't need drawers, and didn't have room for larger desks anyway.
If you are in your own home and intend to stay there, you might consider building two or three 2-drawer filing cabinets and spacing them as you want them, then putting a countertop across the whole thing. That's what we intend to do in our "inside" office (in the house), and I have a similar set up in my craft room. Lots of room....
By the way, welcome to WWT!