I can confirm what Amit said. You can easily upgrade from v6 to v7. And you can connect v7 to ACS (i.e. for now you can reuse some of your v7 data to create emails with ACS, by synchronizing profiles, audiences, etc., using the ACS Connect feature in v7). But there is no migration mechanism from v7 to ACS.
You can either use v7 and ACS jointly via the ACS Connect feature if you have complex use cases that can be tackled by v7 but not entirely by ACS, or you can get a new implementation of ACS if ACS can serve all your use cases.
Let me know if you have more questions or if this is not clear, as it can quickly become confusing,
1. The upgrade from v6 to v7 is partially automated using the upgrade wizard
Yes, except the Resolving upgrade conflicts part, During resource synchronization, the postupgrade command enables you to detect whether synchronization has generated errors or warnings which may result in some conflicts.
You just have to follow the instruction listed here: Upgrading
2. The upgrade from v6 to ACS is in effect a new implementation
As per my experience, it's a new implementation as ACS and classic are two different systems with different underlying architecture., florentlb can confirm the same.
3. In order to migrate from v6 to ACS the customer does not have to goto v7 as an intermediate step.
Not sure, How this works. ideally, it should not matter how are you moving from claassic v6 to acs or v7 to ACS.