Glad to see you chose a workbench as your 1st project. I recommend you stay focused on your choice because it is basic and simple and will be an asset in your workshop. The beauty of your bench is its' simplicity... but if you do chose to use mdf hardboard pictured, do add a couple of cross boards in the frame. This will help curtail any sag. I suspect that next year, you will be looking at your bench and wanting to upgrade it. In the mean time discover what you will want to add (dog holes - vises - t-tracks - etc.). Many of us have chosen to add an additional top layer (could be plywood, mdf laminated board, flooring, or whatever). If you add a new surface, don't glue it on, countersink the screws and screw the new surface onto the old.
Its' never hot or cold in New Hampshire... its' always seasonal.