Apple is much trickier. You can still open iOS projects, modify code, and compile without a Mac connected. However, to actually do design in the storyboard designer in VS or build the package to deploy on a simulator or device you need to be connected to a Mac somewhere on your network. This is apple restrictions and also for the fact that all of the simulators and tooling only exists on the Mac.
You don't have to have a power house machine to do this though, something like a Mac Mini would work fine. There are some cloud solutions out there as well.
Xamarin stuff obeys others: Apple says no dev w/o Apple hardware
If you dont have mac, and belive in xamarin,
My sugestion is try build on android first,
If have good respond from customer, then buy mac and porting to iOS
Porting xamarin android to iOS is very easy if you have mvvm knowledge