The Harbor Freight benches once caught my eye, but they are really pretty light duty things. Also they're fairly low, about four inches too low for me. I like to be able to stand at a bench and let the palms of my hands just lay easily on the top. With a bench this high I don't have to stoop over to work, which saves a lot of back pain. Most kitchen counter tops are close to being the correct work height for me. So, build the bench to save your back.
My bench is 30x72, because that made the best use of the HDF I got. For me, a minimum sized bench is 20x60. I can stand at the center and reach everything on it without stretching or bending over. It's a very efficient size. The HDF top on my bench is 3 inches, or four layers thick.
The legs under my bench are steel from a place called Global Industrial, if I'm remembering the name right. There are knock outs in the legs to wire electrical outlets in. I have, but haven't installed, a set of rollers from Rockler that bolt to the bench legs. They elevate a bench about half an inch so it can be rolled around and lowered again when your ready for action. I really need to get these put on.
I like Masonite for a bench surface. I glue it on, using a router trick that let's me make a bunch of circular little regularly spaced islands in the area to be covered, and then apply dabs of glue to the islands, after which the Masonite is laid on and any excess glue flows into the moats around the islands. It will be easy to pop any badly worn Masonite off and replace it if it ever comes to it. It may sound like a lot of work but it isn't once you get organized. I've built two heavy duty benches putting Masonite work surfaces on this way and so far there's been no need to replace either top but I know it will be easy to do when the time comes. Just slip a chisel under the Masonite and pop it off.
Running thin lines of glue every few inches under a Masonite top will work as well I'm sure.
If you come to attach a Masonite wear surface to a bench with screws, use brass screws to save the edges of you tools.
The work surface can be waxed to repel paint, stain and such.
I've put Wilton woodworkers vises on the two heavy benches I've built, the one I have now and another built years ago. I made wooden cheeks to prevent work from being marred by the steel vise jaws. I'm using poplar now, and some others softwood on the bench I built years back. Some will tell you to use maple to line the vise jaws, but I feel softwood cheeks are less likely to put marks on a project.