Many design agencies do provide a customer with combined packages like design, design + domain, design + domain + hosting, design + hosting + domain + maintenance.
Ideally speaking, I would suggest you divide the packages you provide your client in to three parts
1. Cost of design
2. domain cost + hosting cost
3. Regular maintenance of the site and hosting.
I would theoretically not recommend that you provide hosting based maintenance package and it would be better to leave hosting based maintenance to the hosting companies.
What if the hosting is down in the middle of the night? Or they have email issues. You will need to support the client till it gets fixed whenever they need help.
Further, another disadvantage of having all your customers on a single account is that say if some DDos Strike or virus attack or script attack or if huge site access spike in terms of bandwidth takes place on a single client site it might result in all sites within that account getting affected or going down due to just one site.Further, no hosting company can offer unlimited pack in true sense. All are limited by their small print / terms and conditions where they clearly state that even incase of unlimited pack there are multiple limitations. I also doubt if reselling of shared hosting is legally permitted though many do it, legally and ethically speaking, I would refrain from such an idea.
I have been in to website design and hosting business for over 12 years now, so from my experience here is how I work and quote
1. Fixed cost of design billed.
2. Redirect customer to my reseller hosting and domain registration where I ask client to create an account and buy hosting. Also mention to them that these are annual charges and need to be paid separately and are managed separately by a 3rd Party. If they do not like my reseller hosting and domain registration solution, they are free to move their domain and hosting to any other person freely as I do not directly control them. This also gives them some peace of mind. And also they know how much they pay for hosting / domains. Further, I also mention that all hosting and domain queries need to be directly addressed to the hosting company and yes at times if they have an issue, I do help them by talking to hosting support and getting the issue sorted.
3. Quote a separate price for website maintenance (not including hosting maintenance). For the maintenance amount, I help them add content / update content / any other issues like minor design updates / if there is a script attack etc, fixing that for them and so on. But you need to mention that only X number of things are included and major design updates etc would not be under maintenance.
Hope this helps you decide, how you wish to move forward with your idea.