Experience so far is horrible haha.
Since your new, I dont recommend you get a Dedicated Server since they are not as easy to setup.
I would also suggest you say away from virtualization since it uses up to much resources to run. Also the virtual machines installed on your VPS will run slower than if they were directly installed on the VPS.
If you want to run multiple Installs of MP on one server i recommend using Windows OS, and creating 2 users on the server. Install MP on each user (Dont use the installer version, instead download the zipped version). This way you are not loosing resources from the running virtualization software, and also you are not loosing speeds from the actual virtualized machine.
It depends on what you are doing on the accounts. If your posting pictures on all the accounts + using most tools then you probably will need more ram and processing power. If your just following, liking and unfollowing then it should be fine (Not certain)
Depends if you want a command line version or a GUI version. I personally know nothing about linux command line, so I like Lubuntu Desktop version.
Lubuntu is a lightweight version of Ubuntu. This would use the lowest resources when using the GUI version.
The problem with any OS other than windows is you need to install additional software to run MP since its a windows only software. Adding additional software will use up responses, and also MP will run slower on a virtual machine vs using it directly on the VPS.
Bottom line, its worth it to just get a Windows OS. If you get the latest versions of windows you will get emoji support if you use emojis. Older windows versions will not copy emojies into MassPlanner.