There are a lot of options out there for you to explore when launching your new eCommerce site (ranging from utilizing an open source/self-guided platform to hiring an agency to build a completely custom web experience.) But before you think of the "how", make sure and be spend a considerable amount of time around the "whos" and "whats" of your new eCommerce business.
The type of platform, plugins (including dropshipping) and tools you need to be aligned before launching your new store largely depends on the type of product or service you are selling and who are you planning to sell it to. My company has helped launch hundreds of new ecommerce/marketing sites over last 12 years and without fail businesses who strategically and in a detailed fashion map out who their target market is (and calibrate the front end of their eCommerce platform accordingly) as well as think through the nuances of their production/manufacturing/supply cycle (and have their back office tools be congruent with those nuances) are far more likely to succeed. So in order to answer which plugs and which platform/agency i would start with a detailed laying out of all the factors that impact your business specifically and begin to work your way back to start answering some of those questions. With the entrepreneurs/site owners I work with there is a list of questions you can go through and answer as a starting point, but they most resolve around the areas you'd imagine (target market, growth goals, whether or not there is an existing business attached to it, offline sales, digital and content marketing needs, budget/timeline etc.)
Best of luck and feel free to reach out if you'd like to discuss any items in greater detail!
Firstly, you have to check if selling through dropshipping or whatever is a good choice.
This all belongs to: MARKETING STRATEGY.
It's not about which company is the best or which platform, if you have this problem it's because you lack of a real plan.
You need to take care of Marketing. Marketing is the lifeblood of your business, outsourcing it without even know what's good or wrong, it's like givin the key of your house to the first encounter of the street.
An entrepeneur is basically someone who's expert about marketing and knows numbers.
What is your product? What is your angle of attack which makes it unique?
How much is your budget?
You have to deep define your market and don't fall in the shiny object syndrome: to try everything that seems new or suitable to your business and mistake a tool as a strategy, or looking for the holy grail
Start with a website and social media marketing. Website must be SEO friendly and try using WordPress. Add products on the website and then share on social media. Use paid marketing on social media with advance targeting to boost your posts. After you have traffic start re-targeting the customers using custom audience technique.
For drop-shipping compare the price you are getting with the products already available online. Calculate your profit ratio and then decide about it.
Amazon. Online stores are dying. They are moving toward boutique status whereas Amazon is what malls or strip-malls once were (before Amazon). If you don't know what you are doing don't try to learn it--lean on Amazon and focus on what you do well (product development, branding, etc.).