I think a B2300 (re-badged Ford Ranger) is a great choice for your purpose. They are tough little trucks and easy/cheep to fix when something does go wrong. The decision to leave the spare at home baffles me though if you work in rural areas.
Have you considered building a drawer system to allow easier access to the batteries? They could allow you to have the batteries kept low and at the front of the box during transport without having to get to the front of the box for loading/unloading. If done right it could look very neat and professional with room for your tools etc.
Here is an example
You could build an aero-cap as well without worrying so much about side entry.