100mbps (Fast Ethernet) is just fine for VoIP. While many VoIP phones can passthrough ethernet, I always physically separate the voice and data networks. Not only does this increase security but also avoids having to play around with QoS and VLANs.

A management network with limited access is not a bad idea but this increases complexity. If the business is not large, you can manage the phone switch and associate equipment via the data LAN. Make sure access is locked down.

Go with Power over Ethernet (PoE). This way the phones can be powered by the switch via the network cabling. Saves lots of messy power bricks all over the place.