Always rent a car with a known company.
The cost of renting a car in Pointe Noire is prohibitive. The daily price (for long periods!) makes it more interesting to purchase a brand new car from Europe for not more than four months of renting. So make your calculation.
Also, because of this juicy business, plenty of people will want to rent their own car. Don't make this mistake. First they ask the same price than known companies, and second, you will pay all the normal problems of the car: maintenance, oil, etc. Moreover, they don't always insure the vehicle, so if you have an accident, prepare to pay a new car to the owner: you will be definitively guilty whatever has happen, and if you don't pay, the owner has certainly a frined/cousin/brother in the police or custom who will force you to do so.
The minium price for an old pick up is 100$ per day, and it goes up to 700$ per day fir the Landcruiser
Never take pictures
It is not allowed to take pictures in Congo. Everything might be secret defense.
All public buildings are secret defense, as well as houses of employees of any governmental office. It means that you can be arrested and kept in jail, of course after confiscation of your material.
The best is to have a phone and take pictures secretely.
Once in jail, you need relations to take you out of there, or large amounts of money with you.
Never critisize the government.
Whatever you can think about people in the country, never mention the name of the president, or ministers or members of parliament, or police or their relatives: you might be listened by a cleaner and reported. Then the best that can happen to you is deportation, the worse, nobody knows, people just disappear.
The country has been USSR satellite during numerous years. Still today everybody works more or less for the Security of State. Coming form a democracy, it is difficult to be carefull with our words... in Congo it is mandatory.