You're asking two questions. On the niche v broad question, I agree with the other posters that going niche is a good strategy. You can carve out a space more easily. That said, it may not be super easy to expand out of a nice once you plant yourself in it. You didn't specify the niche you're exploring so hard to comment beyond that. As for monetizing, your second question, a couple of things come to mind such as services like relocation, apartment hunt, taxation and financial services. Visa services could be your lead product, with a lot of add ons one you land the customer.

Outstanding shares are what constitute ownership, not authorized shares. It's unclear in your example what you mean by allocated. If you mean that all 10m authorized shares have been issued, with 8m given to founders and 2m sitting on the company's balance sheet in order to give to staff and future investors, then 10m represents the total ownership and you have to give this founder 1m shares in order for her to have 10% of the company. But, if you mean that the company has only issued 8m shares so far out of the total 10m it is authorized to issue eventually, then that is the total ownership and 10% is 800k shares. Hope this helps.

If you define conversion in the second case as getting an inquiry, then I think that option is going to have a much higher CVR than the first. It's also a lot easier to implement. I would go that route to start—quick and dirty—in order to test the idea and learn more about your audience. If you get lots of traction then it might be worth investing in a much more complex, ecommerce site where users can find art that fits their needs.

My vote is for Wordpress. Webstarts.com looks pretty janky. Also, as Rohit notes, you can really limit yourself with a full-service provider like that. Wordpress has come a long way in the last few years and rivals Shopify in some respects (marketplaces, for example) for a fraction of the cost. Also, the developer ecosystem is incredibly robust. You can find pretty affordable and competent remote devs on Upwork; but it pays to do your own research as product manager and know which plugins, theme, etc. you want to utilize. You can find cheap hosting with the likes of GoDaddy or Bluehost, or go premium with WP Engine (recommended). Hope that helps.

The best way to test an idea is to begin to execute it, which is why a lot of founders say that success is something like 10% good idea and 90% execution. Think of your "overwhelming excitement" as an asset and dive in. But, be systematic and organized. Design your task list as a series of tests. Start to build, but critique at each step. If the idea cracks and falls over, then it was weak and it's time to move on.