Hello,

You got a pretty big question wich is always hard to say. Everybody would say it is hard to tell how many resources you need for the amount of websites you want to host. You are saying that you have less than 10,000 visitors a day. But what is less than? 1000 or like 9000? I suggest, since you want to host around 3-5 sites with opencart, to go with option B.
If you go with this droplet, you can research if this is enough or if you need more. When you need more resources, you can always upgrade your droplet. Thats the nice thing of DO and their amazing services.

Then your second question about the OS you should choose. From my experience I've always seen when people try to host their own services on a VPS without alot of knowledge about it, they get confused whey they use a 'difficult' OS. I reccomend you go with Ubuntu, this is build on Debian but with more ease so you can learn more things about linux and get experience with it.
Also Ubuntu is an OS that is maintained very regurarly with a good amount of updates.

1. I am not sure what kind of linux distro is the most suitable for Opencart, but as I said it would run on Ubuntu and it would be easy enough for you to manage it.
2. It depends on what kind of services you want to run. Some software need a special kind of OS version, but when you choose Ubuntu 15.04 you should be allright and it got all the latest improvements. You don't have to update your OS all the time, this could even cause troubles sometimes when you update the kernel. However there is an expire date of every OS where after that date you need to use some other version if you still want to get official updates. I am still running Ubuntu 14.04 however wich still works fine.
3. People always choose a different kind of setup of their server and it depends on what you like to use and what works properly for you and the services. What I would recommend for you, since you are starting to learn and manage your services on Linux yourself, to go with software that is smooth and optimized right away. It could be a bit more difficult sometimes since you need to configure more, but it gives you a better optmized server. With that I mean you should go with (for example): nginx(webhost), php5 php5-fpm and MariaDB(optimized mysql). As you can see on the official website of OpenCart they reccomend other kind of software. But these packages would work as well and are optimized more. Then you always need to think about security ofcourse, for that you can find alot online but think about your firewall rules (IPTables or UFW), DDOS and brute forcing protection (fail2ban), rootkits, backdoors and possible local exploits (RKHunter) and malicious software protection (Clamav / Freshclam)

I hope this gives you a bit more information, and you got enough to choose your system. If you got any more question you can ask here or contact me on skype: cryptic-designs

Wow, that's a load of information in one reply. Thanks!

I forgot to mention that 10k was my first target of visitors at the moment. Presently, only 2-3k for 1 e-commerce site, but just a couple hundreds for the rest (2 catalog (live) and 2 e-commerce (in development)). All e-commerce sites are SSL enabled sites.

From your reply, I think Ubuntu it is for me then. I initially thought upgrading linux is as easy as upgrading windows or opencart, but I guess OS on server is more cumbersome.

I looked at some screenshot and wasn't quite sure but does DO provide control panel (like cpanel/plesk) for the VPS user? If not, do you know any free ones but reasonably good?

One more thing, my site is currently CloudFlare enabled as well (on the shared hosting). Not sure if I need to install anything on the server or it's as easy as changing the nameserver.

Cheers.