If your countertop has that rounded AND RAISED
lip at the front, look to see if that front edge is glued on and a separate piece--if it is, there's your weak point. So, infill the countertop and install the plywood completely flush with the front edge. Make sure those infill pieces are the correct thickness.
Use a continuous bead of a construction adhesive to make the plywood reinforcer truly continuous. Clamp it as needed, and if you use wood / sm screws to further attach it to the countertop (you should) SELECT THE RIGHT LENGTH SO THEY DO NOT BREAK THROUGH THE FORMICA.
I speak from experience--there is nothing like putting in the new countertops in the homeowner's dream kitchen and having them see the formica 'popping' a hole every time I snugged up the installation screws before I did. Fortunately, her scream--and the 'prick' my fingers received as I reached up to lever myself out from inside the base to stand up--gave me only two spare countertops to take home. And, they were custom-cut, so I had no use for them....
Since you have not built this--consider putting it at a custom height or bar height--e.g., about 42"--depending on how you like to work. I like working at a higher bench when standing (I'm 6'1"), and I like using a LOWER STOOL" when I'm sitting and cranking; It's easier on my eyes, and back, and arms.
If you have not done it, think through and layout your workbench--down to the last inch of length. There are layout drawings on line (architectural standards), and 1/4" graph paper will go a long way to helping a non-drawer. If you really think it through now, You may well sort out all sorts of tricks to help you. For example, build the bench lower shelf heavy-duty, too--and maybe not as deep--it's a good place for bullet inventory. (Better than the 9' shelf).
If one end of the bench is "open"--say by the doorway, consider making the base about six inches shorter and building a 2x6 "bookcase" there for your loaded round boxes.
And on, and on...the general point is, plan it comprehensively now, and plan it all the way through--start with the cleaning location, then move to prep space and then to loading, and finally through to storage.
BE VERY SURE TO shim the press so it mounts 'square'. You could use metal, but a hobby shop will have very thin, small pieces of very expensive plywood that should be ideal.
As for length: My new bench is 4'6" long, and I carefully located two presses on it. It'll work fine for me--crowded a bit, but I don't socialize while I'm loading.
ADDED:. My personal preference is to have a BIG overhang and mount the presses "outside" the base--otherwise, the bench rails can be in the way of press mounting screws and you will have to use lags--which eventually pull out. However, you could (a) mount the countertop nominally 6" from the wall to achieve that overhang, and then build the 2x6 (or whatever) shelving 'behind' it.
At any rate, think it through.
Jim