As to how easy it is to manage your server would depend solely on how well you know how to manage the operating system in question.
A control panel like WHM/Cpanel main task is to allow you to set up web accounts and that includes a web server, ftp server and mail server. Beyond that, it is basically really up to you.
Generally a server is provided to you unsecured, ie the default installation is done and it is not optimized for the web purpose, no firewall is installed and probably no patches done.
With the threats around cyberspace, the issue about security is paramount. It is this and this alone which could make or break your system. Think of it as walking through a warzone. You might not get shot if you are lucky but you get stray bullets everywhere so it is just take one unlucky day and one stray bullet to hit you and you are down. The important thing is to be prepared.
After you get your machine up and running, you do not need to stare at your screen 24/7 if you do not want to. You could set up monitors to check on the health and security of your server. Of course checking up every once a day might not be a bad idea because anything can happen and they would happen when you least expect it.
I had a server years ago where the harddisk just craps out when I was away for a vacation. With slow and expensive internet connections, I had to move files and coordinate with the datacenter to replace the harddisk and set everything back up again.
I am not sure about ResellerWorkz but you could always question them on what is included andd what is not included in their managed solution.
For some, fully managed might just mean that they will keep your system status quo as it was delivered to you and nothing more. Some are proactive which means they will fix issues without consulting you and others are reactive and you need to tell them exact what to fix and how to fix it. Some charges you for these and some don't.
It all depends upon your skills as to what exactly do you know about administering the server. If you are having a good knowledge of the same then you can have a very good time for administering the server. But if you are not that used for managing the dedicated server then you will have to take all the necessary care for administering it.
If you are aware of using the WHM/cPanel then it is well and good. You can install the license copy of that and mange the server accordingly.