Hi Carmen,
I've worked on Shopify for over 6 years including as a merchant, designer, and developer. I have a tonne of experience on the platform and a strong understanding of ecommerce best practices.
YOUR PROJECT
I have read your brief thoroughly. Please be aware that all Shopify themes or custom built themes will be responsive by default and work well across all devices.
It's excellent that you have a list of add-ons. In my experience there are a few related to multi-currency and multi-language that require a bit more research to settle on your best options.
My general process is to do the following:
1) Research: We would understand your products, brand, and target audience.
2) Planning: We would settle on your content and functionality requirements. In particular we need to understand any complexities with your products. Whilst doing this we would refine your design preferences. I understand you like a minimal and monochromatic look.
3) Implementation: All the coding would be done here. Based on our research and planning we could either choose to use an existing theme and make customisations, or we could build a theme from scratch completely tailored to your brand. This would cost more.
4) Refine: We would thoroughly test the site and implement a final round of revisions to ensure you're happy.
It would be great to discuss this further. If you are interested then I'd love to hear from you. Otherwise, best of luck with your project.