What are Patching & Hotfixes and Spam filtering?
Why I don't see features in managed services?
That's when your host ensures your server software is up to date by installing the latest version, fixing any known issues, and plugging and exploits or gaps that are known or bought to their attention.
Not sure what providers you're looking at that don't list these things, but worth getting in touch with some to find out exactly what they offer and don't. As I mentioned before, there is no standard of management, and with some providers theres many levels.
Hotfixes, patching your server and helping with spam to your server is just things that we include, so I can't say for anyone else.
I am not sure I can manage a dedicated server completely like a web host provider but I can make a website online running from an unmanaged hosting.
Is that enough?
From what I get from what you said, are you basically looking to get a Dedicated Server so you can sell web hosting yourself? If so, are you sure you need a managed service? Unless you prefer the peace of mind of not having to manage yourself. You can provide hosting to your customers by using WHMCS & WHM...