Yes, DF is fine. So are yellow pine, white pine, ponderosa pine, spruce, hem-fir, alder, poplar, and pretty much any other kind of wood you can get your hands on inexpensively and locally. Choose the wider 2X boards (8s, 10s & 12s) instead of 2x4s (which will have more knots and defects that can't be sawn out), and rip your pieces from the wide boards. You'll get clearer pieces that way because you can cut around knots and defects.
Alternatively, don't laminate anything and make an English style bench from full width kiln dried 2X stock. I used 2X12 hem-fir, a couple 1x8 white pine boards, and two 8' DF 4x4s for the legs for my [third] bench. All told it cost me less than $100 for all the lumber. I built it entirely with hand tools, from start to finish, in about 3-4 days. It's the best bench I've ever used. Better than my first laminated bench (built from 2X material), and better than my second bench (laminated from birch, my least favorite of the three). But more importantly, it was the best one to build. It took the least effort, went together weeks faster than my laminated benches, was the easiest to flatten the top (because it wasn't laminated), and overall was much less stressful to build. I actually enjoyed building this bench. I can't say that for my other two benches.
Save the nice expensive wood for your furniture. Workbenches don't need to be made from fancy, expensive hardwood to work well as a workbench. You just need to be able to build them quickly, solidly and easily, and to suit the type of work you do.
Here's a podcast series I did on building it in case you are interested. Have fun!
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