1. In place upgrade - yes. When you pay for VMware ESXi, you'll get a license key, and when you add this license key to the server, it will turn on those features you've paid for.
2. If you've only got a single ESXi server, it would be slightly over kill to use vSphere vCenter, although you would have the benefits of creating templates for quick server deployment.
3. To be able to use vMotion you'll need to have shared storage, it depends on what requirements your small shops requires in terms of availaibility and resilience etc.
