I am looking forward to seeing what other members of the Community could add to this, but your question has my brain buzzing so I wanted to get down my thoughts.
There are some "limitations" to keep in mind. First, it's how your email will display on desktop vs. mobile and even between different email clients (though in this case, it's usually the same across the board).
If we're talking strictly desktop, our email editor limits columns to 3 per row. So, if you have 8 premium businesses, it would be four rows of 2 columns. For the 10-15 basic businesses, it would be five rows of 2 or five rows of 3. Admittedly, this would make the email incredibly long, but if it's important for each business to occupy its own space, we just have to accept it.
Design-wise, each 2-3 column row would share the same color, so I recommend choosing a color for premium and a color for basic and then alternate between the chosen color and an alt color (or white). By that I mean, the premium and basic businesses will be separate, but within their sections you'll alternate each row with a different color/off-color. How you depict each business is up to you, but I would assume something like the business logo/business title/link to their website is sufficient unless you wanted to include a short blurb for each, but that will continue to increase the length of the email.
I am rubbish with color theory, so it's hard for me to recommend the right color schemes but I would say either base it off your brand colors or do some research on color theory with a focus on complementary colors. For example, blue and orange are often used together (but might not be applicable here).
I would also add separate blocks above each section that introduce this is the premium businesses and this is the basic businesses (however that would be framed).
The below example is a hastily made idea of what's in my head (missing the introductory blocks, though). You could also save text by linking the website URL to the image, but also keep in mind that image-heavy areas can kind of blur together, so think about how text can create appealing contrast.
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