1. The Reynold's number will be calculated based on the values of density, viscosity and the geometry of the model. If you are new to Fluent and if you have to choose a turbulence model, I would suggest you to go with the default settings. To know more about the theory of turbulence models, please refer to the links below.
2. To answer your question on the number of iterations.. It depends on the problem and the accuracy that you require. For smaller residuals, you will need to run more iterations. Another way to monitor the convergence is to monitor a parameter of interest in your problem. To know more about it, please refer to the link below.
3. Can you please give me more clarity on what you want to know for your third questions?