You can check out
www.planequest.com they have a lot of good information on operating cost of many types of airplanes (US dollars).
A twin Cessna would be better than a Navajo. Its easier to get parts for a Twin Cessna then the Navajo. It would be still better to have a King Air, best would be a light jet, if you have the money.
Look to see if a charter company is for sale in your country.
It would be far easier to buy a existing company then to start a new one. Plus if you need a business loan its easier to get a loan for a existing company.
JH