I would agree with the majority on a good billing system. it is a #1 must have, to allow you manage your clients better. Since you are going to be either alone or working in a smaller scale, your time is your asset. You can't go around wasting time managing inefficiently.
I think the first thing a reseller must have/do is to research their market, I have seen too many thus far ask for advice only to purchase the highest cost servers simply based on the processing power they currently use on their desktop. Putting that into perspective an I7 Dedi totalling £278/month forgetting then licensing (cpanel/plesk, windows/linux (some), whmcs/modernbill).
The reason I point this out is that based on most common packages between £4/8 (pm) to pay for all of the above listed there would need to be a high volume of custom meaning that most newer companies would not have the capital unless eating into startup or additional funding especially when marketing and other costs are added.
It’s become one of the many things I now look for with the specs provided by hosts (mainly newer) when researching or looking around both to monitor trends but also it can play a part in the support performance as 24/7 does incur a considerable cost.
Apart from that ensure you have all the more unnoticeable things setup such as:
SSL (clientarea’s / support / control panels and also shared)
Terms of Service / Acceptable use Policies / Privacy policy (clearly visible and available, also ensure that your billing system ensures that your client agrees to the terms of service (this will then include reference to the AUP / Privacy Policy)
W3C valid (and visible) not necessary but it is always an indication that time has been spent ensuring that your website (brand) is as compatible and to standard as possible.
Just a number of small things that are easily noticeable but also easily resolved, though there are many others
Being a hosting provider is not at all easy. You have take care of every issue that the client faces. You should be pretty sure that your support is available 24x7x365. The support must be available by every means i.e live chats, tickets, mails and phones
A knowledgeable of clients support specialist team backup all the hosting related issues for your reseller hosting business are one the mainframe of your business.
Reseller isn't the easiest to jump into right away. If you shared hosting is growing well I would suggest getting into it. So far the biggest things I've seen that people want out of resellers is space and what billing software will you give them. Everyone is different.