I have heard talk of this before however I have a simple solution:
Active Directory is one service that should not be place in the "cloud". It is by far the hardest service to implement as a cloud environment simply because it sits pretty close to the network stack as it is.
- DNS Lookups - You would have to route all DNS lookups to your cloud environment first to look for resources and then do web lookups for everything else. If your service is offline they have no internet, that would really make some customers mad.
- John mentioned some licensing issues above, you'd have to have a lawyer review the EULA from Microsoft, expect some major cost here.
- You would have to convince your potential clients that their information is secure. I really don't know how you convince them of this.
Alternate Business Plan
Instead of trying to build out an environment of your own for each customer why don't you simply combine multiple service offerings into one. For instance:
- Buy and Install a Tower Server to run internal services such as DC, DNS, etc. This helps you manage usernames and passwords. There is nothing preventing you from buying a server and OS license and then "renting"/"leasing" the hardware to another business in a subscription model. If they decide to leave you come claim your server and be on your way.
- Don't even worry with a server... simply setup Microsoft InTune on their Windows workstations and use that to manage usernames, passwords, etc. You can even manage the entire update and maintenance from that client as well.
- Leverage cloud providers for collaboration tools. Office 365 would be a good start. More recently Microsoft announced new billing mechanisms for partners that will allow you to directly bill clients so you can bundle other value add services along with both the InTune and Offcie 365 offering.
What does this mean?
Essentially you utilize the resources of other providers, Microsoft being my example, and add value on top of that, be it customer services, help desk, whatever. Regardless all of these solutions allow your customers to grow and scale and allow you to be active in their technology decisions.
Regardless I would not try to do any AD stuff remotely or over a VPN you are simply asking for trouble in more ways than one.