What can you tell us about the server specifications, and the amount of traffic expected for the website? You could configure the the server to meet all of the minimal requirements, only to find out it fails to function properly due to an inadequate configuration (or another cause).
It sounds like we are dealing with an older dedicated server, or perhaps VPS server that you have root access to adjust the configuration. Generally this points to having plenty of resources, granted you should also ask how the server is being used (mail, dns, multiple websites, other services?). Susan is correct, in that you can use yum to install missing elements, however you need to understand if adding ImageMagick will interfere with another service. You might have a different image processing subsystem that can't coexist.
That being said, you also want to look closer at the key components to ensure they are current. If the server has php 5.2.x installed you can technically run MODX, however it might not operate properly as some extras use functions found in PHP 5.3.x or 5.4.x. Not to mention the security risks associated with running EOL (end of life) software versions.
It's great their support is helping you, but one thing you should keep in mind - most support is reactive. They will only take action(s) when they are informed of a problem. If you need help changing the configuration for example, or if they get reports your server is compromised/sending spam. They are also going to use more general configurations, if you need server specific help regarding MODX, you will want to work with a strong MODX hosting provider. This is where dealing with proactive/fully managed hosting can be nice as they will work with you to better avoid problems.
Oh, getting back to the original question, you can get away without those missing components, but I would strongly recommend installing them for the best experience with the system. The package manager is far too nice to not use because you are missing cURL/zlib
I hope that helps some.