Hey, I don't write long copy pasted messages. You took time to write this, I'll take mine.
There are so many things to discuss since you've given up so many details! The look, CSS part is easily doable, I can easily copy the header too.
Integration with POSIM is also easy. Popup is fine too. Discounting, limited stock, all are basic features and can be done and taught how to operate.
All the apps you mentioned have been used by me on previous projects.
There is not a single line there which isn't possible IMHO.
Let's discuss and start.
I have ONLY 5-star feedback from my 70+ clients here on Freelancer.com! I have 5+ years experience in Shopify API too. So, whatever isn't possible in the basic customization, I can easily do it with the API too.
Let's talk more,
Nitish.