I've built three darkrooms. In the first, I made the counter from formica over plywood. In the second, I used particle board with several coats of polyurethane varnish. Actually I really liked this - it had the warmth of fine wood, but was far less expensive. In this design, the enlarger was mounted on a small wall shelf, and the counter area under the enlarger was rigged with a drop shelf arrangement to permit bigger enlargements. (I had a low ceiling, and couldn't raise the head on the enlarger all the way to the top of its column.)
In the most recent, the enlarger station was in a corner, and I used the varnished particle board for the area under the enlarger. I got around the enlarger height problem by building a recessed area in the ceiling above the enlarger to permit the head to go as high as it could on the enlarger column. Then, I put inexpensive commercial kitchen cabinets (from Lowes) on either side of the enlarger station, and commercial "postformed" countertops (actually, formica over particle board) over thsoe cabinets. We have a discount building supply outlet that gets odds and ends of countertops - simply selected on in a color that I liked, schlepped it home, cut in into two pieces, and mounted one on each of the cabinets.