Not intending to sound crass or arrogant, but there are no suggestions or "instant-intelligence pills" to offer you that will ease the strain of learning a new logic and for the light to go on a bit brighter when it comes to understanding eCommerce, web building, or planning a business model and translating that onto another channel (the web).
Try starting from the bottom rather than the top: if you have a Busniess Bank account, you probably already have a Merchant Credit Processing relationship, right? Call them as find out which gateway they work with, so you can verify which "cart system" to focus on, as not every system deals with every gateway.
Since the PayPal Pro account (which allows you to process cards on your account within the cart rather than on the PayPal server) is about the same price monthly as a regular business setup, I would suggest sticking with your present bank relationship and go this way.
Once you are square on the payment methods and structure (which helps to narrow the field), concentrate on the merchandise line(s) itself: do you have good images for each item prepared with item number as image title, all the images the same size and resolution, and organzied in simple folders to make managing them easier from the get-go? Do you have all the attributes (item number, vendor, cost, retail price, sell/discounted price, color, shape, individual product cart/showcase description, etc.) for each item also organized in a universal format in Excel ready to be loaded into your cart?
The attributes are usually columns across your sheets. Knowing how many products you actually intend to present, and the structure of Categories and Sub-Categories all have a big part to play in not only which cart can accommodate the cat's and sub's, but in how you will need to configure your navigation.....
Speaking of loading .... you will need to load certain page content (like Privacy, Policy, Terms of Use, About Us, Contact, etc.) in addition to whatever you are required to publish, so this can be in development as you continue to shop also.
My point is that there is a lot of work to do other than stress without knowing exactly what you think you need to do, and in fact, the grunt work is what takes the longest. And I know that once you being pencilling in your real web plan, your mind will constantly gravitate toward trying to piece together all the different elements, which is a natural way of learning a new logic (and you thought it was just because you wanted to get it right the first time and not do it again, right?). You will become more aware of how linear the process truly is, and you will notice each element as a natural part of your plan as you design your navigation, your inventory organization, and even how to present your images and graphics.
If during this work phase you continue to "shop" for a template that has the same layout you are leaning toward, and remember that you can easily change the colors of backgrounds, borders, etc., then you will feel more comfortable with the whole system, and again, see more things that you can make work for you.
It is best to find a simple template that needs very little color changes and such, as often this becomes the boondoggle to snare many, so concentrate on a workable layout and basic color changes first. Also while shopping templates, look at each of the sample pages and how they work, as this will help you to see the logic in play (and what pages you need to develop and what others you may 'delete' from your initial plans). It simply makes it easier to understand the basics, since that is what you need to master if you are going to do the updating yourself.....
It is not difficult to learn, but does take time to feel comfortable enough to carve your own groove in eCommerce. Most of the work is not in the design, but in the organizing, preparing, and formatting. Trust me. New ways of seeing things along with a completely new, rigid, and unforgiving logic is like learning a new math years after graduating. Not impossible, but you keep asking yourself "Why?"!!! LOLOL
Bottom line? Change your perspective, not your expectations, and you will emerge empowered to be "eCommerce enabled" in no time!
OK...here comes the pitch: if you find the task simply too much a bother and would prefer first-class results and a set of easy-to-follow crib instructions to make "managing" an eCommerce solution easy and painless, I just happen to specialize in this service, available at
www.ProWebsUnlimited.com
Also, you may wish to simply browse for an eCommerce template
here, and once you decide on a look, I can make it available at a discounted price.
(Even though the interior Content pages of the site is still being updated, the template selections work just fine)