An M-Leica is an amazing shooter and if all you can afford (all!) is a Zeiss lens, then get one, it will be wonderful, Zeiss makes wonderful glass.
A new zeiss lens is going to be better, optically, than an old Leica lens -- better coatings and computations and so on.
Bottom line, get what you can afford and shoot. An M3 would make an excellent and much cheaper choice that an M4 P, for example.