Hi David,
In my opinion if you want to customise your site to be unique from the rest of the other site, you can draw out your site. Produce your own control panel or integrate other control panel within your website.
This is really hard to explain on one forum post. In fact this can be a one chapter book. I can however simplify this down into a general term. All control panel must interact with your browser through POST and GET request. Thus if you can program your site to use PHP, Ajax, Perl, CGI, Python. You can have a website script automatically patch into any control panel platform and create or delete account on criteria. This is limitless. You can secure the connection between your website and the control panel server using SSL. If you are more advance you can use IPSec tunnelling which will produce double level security. However this will require a lot of configuration. This however can cost you zero dollar on setup if you know how to do it all.
The other viable solution is always, purchasing a control panel software from someone. The popular packages that I seen lot of people mention are Cpanel or Plesk for server management. WHCM for billing or you can even use Box Billing, to be honest any digital billing software will work as long as it can let you have a hooks call depend on how the user are interacted.
The other solution are always that if you purchase a re-seller account, the hosting provider above you probably offer a white-branded solution. Which mean you can give them a template and they will customise the platform for you base on the template. However these type of account about 10 years ago can cost up to $700 dollar. It must be a lot cheaper now since everyone compete for client base and auto process by script also made everyone life easier.