MDF suddenly give way Bazyle ? Me thinks you have never used it. The only problem I am aware of is that for 8ft long shelves with plenty on them they need 4 supports to prevent bending. I can only get the nominal 3/4 inch stuff. It's still stiffer than the same thickness of ply.
Legs – take a look at some racking units with well over 100kg per shelf loadings. Really well over in some cases. Pure compression so not much is needed – just 048 thick steel angle. Rather than trying to cut sheet precisely it's much simpler to sheet over the sort of construction I posted which also has the advantage that the only thing that is critical is the leg length. If incorrect the bench will rock.
It's easy to prevent it bending on something like a bench top – screw and glue some strips of it underneath. It glues extremely well but the smooth finish is best roughed over a bit. Cut edges glue very well indeed. The deep rear section I added is part there to resist bending. I've stood on it – not a good idea. I've some taller steps now.
I need a mk2 small bench for my "new" workshop. There isn't room for the one I posted as it's 44" long. I've part taken up the space for something bigger with more racking. The 300mm deep racking is very useful in a workshop. The deeper stuff tends to hide things at the back as it gets loaded up with more bits and pieces. It's best to buy an extra rack unit to gain shelves. I have high ceilings and have used part of the spare rack to extend the height.

I have plenty of very heavy stuff low down but the sections can be clipped to the wall. I currently have a mini lathe in the wide gap. It's on a steel tray. With some thought It would be equally possible to add a deep bench to one of the shelves to carry a lathe if needed.
The only problem with these racks is the height taken up by the shelf supports. Some are thinner and the chip board shelves are inset rather than just sitting on top. The supports are also thinner. They do cost significantly more than the type shown though, This is an example. Other heights are available but 2.4m is just about right for me.
**LINK**
John
–