We don't keep a lot in the cab but we do have the builder mount two litebox's between the forward facing seat and door on each side so we can grab them on the way out or reach in and get them. We supply the litebox's to keep it fair to all bidders, they just have to mount and wire them.
Have the builder add a hook at each position for the headset, mask, portable radio on a strap and possibly road vest. It will give everything its place and keep it from getting tangled up.
We supply the radio, have them mount it and hook up the antennas. We usually have to plug the power in for 'final' installation but then it's ready to go.
We supply the thermal imager, MDT mount and Knox box but have them mount and wire them. In the past we had a plate that everything was mounted to and the plate mounted to the dog house. After seeing it on a St. Louis truck we put a plate in that was hinged on one side and latched on the other. Your electrical boys will love you.
We put at least one Kussmal dual USB port in.
Some chassis builders can slide the crew seats back and cant them some for improved leg room.
Our new rescue has an EMS compartment inside for our BLS bags and gear. It's on the back side of the doghouse and we mounted glove holders with all the sizes we carry on top of it. Also have a 110v outlet there for possible future need,
We also have uni strut mounted horizontally in the back of the compartment similar how it is for pack brackets. We put a vertical one on the end like for a shelf. Then you can put metal or wood on it to mount tools to and mount the metal to the uni strut. You lose about 1/2" to 1" of compartment depth but it makes the compartment versatile for the life of the truck and you're not drilling holes in the truck.
Like the stuff in the cab, it adds minimal cost to the build but is worth it as equipment changes over the years.
Hope this helps,