The answer to your first question about hiring an employee is one of those annoying "it depends" answers. But having said that I'll try to share some best practices.
If they're going to be helping you fulfill orders they need to be detail oriented. That means that have to be paying attention when they're packing things up to make sure every box is packed correctly. In a warehouse you want to have 0.2% or less error rate with your orders. If you have a bigger error rate than that something is wrong with your system or with your hire and you need to fix that.
If they're helping you with customer service they should be be empathetic. You can teach people technology you can't teach them to be a good person. It really depends on what business you're running but you mentioned Zendesk. I use Zendesk and it's a bit heavy. It does everything you want but might be a bit clunky. If you're just doing email support HelpScout is really simple and usable.
If they're helping you with marketing they should know a little about SEO, content marketing, adwords, newsletter marketing, & social ads. The reason that they should know about all of them is that you don't know which will work for your business until you try it. For one business content marketing will be amazing and for another it won't do anything. I would avoid hiring someone who's amazing at just one of those until you prove that it is a viable channel for your customers.
For an e-commerce platform I tend to recommend either Shopify or WooCommerce. Shopify is hosted which means it's less customizable but you don't ever need to update your software. WooCommerce is open source which means you can literally change every line in it if you want. You can customize everything but the trade off is you should spend at least an hour a week updating the software on your site. I only recommend Magento if you have a huge budget because you need a developer to do most things for you. Both Shopify and WooCommerce are simple enough for someone without a technical background.
If you're integrating with Amazon or other marketplaces like EBay then I recommend you use software that integrates your sites like ShipStation.
For warehouse technology if you're shipping products there's a fantastic app called ShipHero which will track everything for you and give you stats per employee.
I hope that helps! Any more Qs feel free to give me a call. :)