Asking an Tour Operator probably doesnt really help for that purpose.
In my prefered International Car Rental Provider:
https://www.billiger-mietwagen.de/
There are cars between 30 and 130$ a day. No driver. But no overlanding equipement too.
Guess if you walk through Ndjamena you would find more options.
How I usually do
overlanding by rental, you can read here by Chrome Browser and translation from german.
This way I did overlanding in
Laos, at 2010, where you was able to read anywhere that you need a driver. I was able to do selfdrive that way.
So for those who wants to go soon, it may be worth to call the local Avis rental, and ask for the requrired paperwork. Please give us an update here!
Surfy