Personally, I recommend that you keep your web hosting site seperate from the customer sites. It's not about trust, but rather reliability. Hacks and spams DO happen - last thing you'd want is if your system was routed, that YOUR site would also be in the group. It's stressful enough to get your customer sites back online, you don't need to be worrying about your site too (in order to communicate with them).
Billing should definitely be on a different site/server.
If you're on a VPS or Dedicated server, there is no issue with using the default setups for packages etc, you should not need to create a reseller account for that. Creating a "root_budget" package and then assigning a user to that will not create any more security issue than having "username_budget" as the package name. It's just a matter of how it's viewed in the WHM interface.
For private nameservers, you can create your own on the VPS or Dedicated server for your customers. You can then have your own nameservers managed through your domain registrar if you wanted.
You COULD setup a hosting account for your own website with another hosting account - get the minimal hosting package, and keep the site separate from your customer sites. This would be a cheaper route in keeping things separate (rather than having to purchase multiple VPS packages/machines).
We use 6 different servers for our own site - everything seperate from each other so that customers are always have the ability to be in communication with staff. With your own customers you should be running TWO seperate machines for DNS and not using just your primary server for everyone to be on.
Also, be aware, if you are using your own server and put your account on the same machine as your customers, you MAY run into the situation where you are unable to contact the customers (should the drive fail etc) - or you may have the problem of mail being routed directly to accounts on the same server, rather than to a remote place (say they were moving from one host TO you).
These are all things to keep in your mind as you continue the expansion. If you have any questions, I'd be more than happy to discuss possible solutions with you.