I would be cautious about having a billing system integrated with the control panel. I know that Helm has a built-in billing feature; but the problem there is that if you start using that, you're either tied to Helm for the rest of your life, or if you ever switch it will be a bugbear.

I have always been of the mind to have my billing system, site CMS, and control panel separate. They work with each other, but I can change one without *having* to change all three. It gives me more flexibility.

In answer to some of your questions:

1. both Plesk and CPanel can be configured to "jail" a user's account so that a user can access system libraries and resources, but cannot access another user's account space. I don't doubt that other control panels do this, as well - but I have actual experience using both Plesk and CPanel, so those are the only two I mentioned.

2. Helm runs on Windows, and does have integrated billing - but see my previous recommendations about keeping billing and customer management separate from control panel software. (Your mileage may vary.)

3. For ease of use by end clients, I prefer CPanel, followed by Plesk. However, as CPanel doesn't run on Windows currently, I think Plesk will be a better fit for your situation.

4. SSL certs are not required to use CPanel - just advice to the clients that the error dialog is expected, there will (of course) be no secure certificate for their domain control panel, just click "accept" and log in. I don't know about Plesk or Helm - I leave that up to the folks who administer the servers. Ask the control panel vendors, and they may be able to clear this one up for you.

5. I don't know. Another question for folks who are more hands-on with server administration.

6. That depends. I personally look for a datacenter that has more than one location, that has an excellent disaster recovery plan (for their clients as well as for themselves, if you plan to co-locate), that has been around for a while, and that has the capacity to handle growth quickly and efficiently.

7. That depends on the server's capacity and your clients' requirements. If you have a bunch of clients whose disk space and bandwidth usage is fairly stable from month to month, it will be easier to figure out how many people can go on that particular server before it gets bogged down. Again, if you have three dynamically generated sites with huge amounts of traffic, they may need access to more resources than, say, 50 accounts with static pages, some image galleries, and not a lot of traffic.

8. I don't know. I've seen people asking about that, but I've never offered free hosting or placed ads on my clients' sites, so have never researched it that much.