Here's a couple things to think about.
I see you intend to get a 5er and pull it with an SRW truck. Then you might want to take a hard look at the payload capacity of your truck. Take 20% of the GVWR of the 5ers you are looking at (don't use the dry weight). That will be your estimated pin weight. That's the number to use when determining how much payload you are going to have. Also add about 200-250 pounds for the hitch and the weight of passengers and any other gear. Then add the weight of the tools you intend to take along, plus the weight of the box.
Is your truck a diesel ? If so, then you may want to carry extra fuel, which could complicate things. I have an RDS box that I like very much. It's a combo unit. Tools in the top compartment (6 inches deep) and 40 gallons of diesel below. I suggest going to the RDS site and poking around. They have all kinds of boxes.
But the fuel alone adds significant weight. I carry most of my tools in the basement of my 5er.
If carrying tools, I'm sure you want a lockable cover, so you will want to pay attention to the height of the box, making sure it doesn't stick out above the bed rails so you can put a cover on the bed. Many hard covers will clear my box (which is actually slightly below the bed rails), but I ran into problems because so many need clearance under the cover for the folding mechanism. I ended up getting a rollup flexible fabric cover. At least prying eyes cannot see what's under it. It can't be opened without opening the (lockable) tailgate. Yes, I know what a knife would do.
Full disclosure - I fulltime. And my TT is a dually so I really don't have too much of a payload problem.
And thank you for your service !
Tim