Impact of adding standalone ESXi host to cluster

The VMs will keep running, there will be absolutely no impact on them.

The VMs will automatically show up in vCenter. IIRC you can select a folder where they are placed.

For the host, it depends on where you place it in vCenter. If you create a cluster in vCenter for it, it will keep running, just with central management. If you add it to the existing cluster, it will be treated like all other hosts in the cluster, with migrations (if the CPU is compatible).