An M3 would make a better center for the M50's the way I understand it. They are somewhat sonically matched.

The best center for the M60's would be the VP160. Second best would be VP150. Third best VP100.

It all depends on how much you want to spend and what you have the room for.

You can also just skip the center and go with the M60's for now, then add a better center later when you can afford it. You're AVR will provide a phantom center, so you won't be missing any dialog.