If you own a web design company then you obviously do not need to pay for the website to be built, so you're half way there.
The thgings that I suggest you put the most money into is this:
1. Your reseller account or dedicated server. For this I suggest
http://liquidweb.com . They offer a great VPS package, and there dedi server plans are great! If you think you would rather go with a reseller account, then you can have a look at our reseller packages, some include free end user support! At any rate, whom ever you choose, do not look for price alone, as you will only end up paying more for the errors. Just as the old saying goes, "Cheap is usually more exspensive".
2. Your billing system. I have used alot of different billing systems including WHMAutoPilot, ClientExec, ModernBill, iHost, and couple others. Currently I use WHMCompleteSolutions. You can get it from them at about $14 a month, or you can buy it from a reseller for about $10 a month. It has everything you would need to run a great hosting business, without all the confusing extra "stuff" that ModernBill has. ClientExec is another good option too, I was very happy with it. WHMCS and CE are the 2 most user friendly billing/client managment systems I have used.
3. The support for the clients. If you go with a reseller, look for one that provides end user support as we do. Thsi will save you alot fo hassle in the long run. If you choose a host that doesnt offere end user support, then I suggest bobcares.com. They offer a great package for resellers (up to 12 clients) for $25 a month. I have found that nothing attracts clients more then good price, and great support, so you'll want to invest in 24/7 support for sure!
Anyway, I wish you the best of luck. It's not an easy business to make it in, but if you try your best and put the money into it, you'll do fine after awhile.