Whats the old saying?
"The only way to make a small fortune in aviation is to start with a large one"
If you have passengers already lined up you must have an idea what aircraft they want to use. As much as I love piston twins unfortunately things move on and your average potential passenger will be used to and wanting something a bit more "modern" than an Navajo. Unfortunately even king airs are frowned upon by many. Which is unfortunate because in the UK it is probably the most practical aircraf available!
As has already been said if you are low on hours, take your passengers to another company and get some experience. Then think about going on your own
