Hi,
you will need to register company, and that will provide you papers for legal stay in Montenegro. You will need to rent office for company, price varies, depends what you find, you can even find some office rented to somebody else, just to cover you, since you really do not need one. Monthly expenses, besides office, will be deductions for you as director around 120e and accountant 70e. For opening company and your papers for legal stay you will need aprox. 500-600e to pay to lawyer or agency who will do it for you, plus 100e taxes. Later on you will need every year to extend it which will cost 100-150e or 50e if you want to do it by yourself.
There is no any kind of restriction for your car hare.
cheers, and welcome