Since you are just starting work with a cargo carrier just a couple of words to take heed to.
First off, don't discuss your plans to bring a handheld GPS on your flights with management even though the aircraft are minimally equipped. Handheld GPS'es are not certified for use for IFR as I am sure that you are aware. Management might not be too accepting and may even have restrictions on their use. If this is your first 135 job which I sense it is, things are run a little differently than Part 91. There is a lot more oversight and regulations. Try to keep your nose clean while you are first starting out.
Someone here suggested trying to persuade management into having some fancy piece of equipment installed. Cargo Operator - forget it. Your lucky he doesn't have you sitting on a crate, or better yet a box that's part of your cargo.
Secondly, depending where you are operating, how long your legs are and if you will be flying during highly congested times/airspaces the handheld is more a nusiance than a help. It can become a dangerous distraction.
If you do use one get something simple that will allow you to change routings and get DIRECT TO entries with the fewest keystrokes possible. I had a Garmin 295 and used it as a backup on a cargo operation. There is so many keystrokes necessary for input of information and building or changing routes that it just about made the d*mn thing not worth the bother.
In the cargo realm of operations what the GPS is good for is giving you situational awareness, assistance in tracking in between VORS when situated a long distance apart, groundspeed readout, and distance/time to your destination. You may be more apt to monkey with the unit when heading to your outstation which in most cargo ops is a small municipal airport. When you head back to the hub or your base which in most cases is a large and very busy airport, before you get into the terminal area turn off your handheld, it will just be a distraction.
If you can maintain radio contact remember that you are " /RV" (Radar Vector Equipped)
Just remember to practice KISS! (Keep It Simple Stupid!)
Good Luck and Congratulations on your new job.