ConcreteCMS is a fantastic CMS, so you’ve certainly made a great choice 
There are a few options for eCommerce on Concrete, with something like Community Store being one of them.
What I’d perhaps just caution though, is that whilst Concrete is very easy to pick up and learn how to edit and manage, adding eCommerce considerations on top of learning the CMS, could make the learning curve altogether fairly steep, especially if you’ve never used a CMS at all before.
I’m the main developer for Community Store, and I’m pretty happy with what it’s become. It can be installed into a fresh install of Concrete, and will work with either default theme, presenting a neat and fairly feature rich eCommerce platform, where most configuration can be done via the Dashboard. I’m also doing my best to keep it updated.
That being said, like all software, errors can still occur, or you may need to make some coding changes to get things how you need, and it then comes down to your troubleshooting and technical skills to work out how to resolve things. The forums and github can help with those things, but it’s worth considering whether you want to take on that longer-term responsibility.
It’s also an add-on that you’d get the most out of if you’re reasonably knowledgable with Concrete’s concepts and approaches. It tightly integrates with the CMS. I’d be suggesting that you get quite comfortable with Concrete first before moving onto the eCommerce side of things.
So it’s worth taking a step back and considering things like:
- are you just selling one or two things, or is it for a large catalogue, with lots of categories?
- would it be business critical, i.e. if your site needed maintenance, would it be low stress, or would you potentially be losing many sales?
- do you have any complex needs, like unusual shipping calculations?
- do you have complex tax charging requirements?
If you’re only selling one or two things, Community Store might be overkill, and you could end up spending a lot of time on getting it all set up right, to then not need all the features it has.
One option worth considering is something like my Shopify add-on:
This allows you to use an inexpensive plan on Shopify (their ‘Lite’ plan, one lower than then basic plan, $9 USD a month from memory), for the cart and checkout side of things, as well as customer and product management. But you can then link that Shopify account to your Concrete site and place some very simple add-to-cart buttons, as you would other types of blocks to a page.
It’s a solution that you can actually set up and get working within about 15 minutes (see the video on that marketplace). You then can just build your site however you like, describing products with normal blocks, adding in the Shopify buttons where needed. You then manage all the more complex eCommerce side of things on Shopify, where their platform is really very slick (and reliable).
I’ve used this solution myself with a few clients, simply because it’s been easier and quicker to set up, and/or they’ve been interesting in using Shopify to manage their orders and products. We get the best of both worlds really.
We then use Community Store were we need to do a lot more customisations, or Shopify just doesn’t quite have the features we need.
There are some other options available beyond those two, but I’ll not go into those here.
Just some things to consider before you jump into it all.
(I did note your specific questions, but I think it’s worth considering your needs and goals first, as those considerations actually change the answers to your questions!)