If all you need to know are the other factors that could be taken into account then this isn't really a maths problem it's a business problem and if you are working for the firm you probably know more than anyone here.
As far as the maths goes, since vehicle cost is a lifetime cost you want all the cost and profit figures to be related to the lifetime of the vehicle so you should not use annual or monthly costs. If you have information for the maintenance costs per month then you should convert that into the maintenance costs over the lifetime of the car.
Other than that there is a small mistake in your formula, the term "Rate per day * no. of rental days" should be N*(Rate per day * no. of rental days)