Hi,
I am a 6+ years PHP, eCommerce expert, looking forward to work on your project. I've a small team of expert developers with the ability to fulfill this big project with perfection.
1. Yes, phalcon is pretty fast framework however implementing good eCommerce in phalcon is little complex. You may also try WordPress with custom eCommerce, its a bit slower but feature rich.
2. However you have a pretty good budget range, so I think one of the best option will be totally custom PHP based eCommerce.
3. Yes ofcourse for that range of visitors, you'll have to use SQL optimization, caching, CDN etc.
4. And for easier maintenance of the vast system like this, logger, tester are also needed.
However, I would like to discuss further about the project, knowing the type of products and features you want, I can help you more precisely. Feel free to discuss further about anything, anytime.
Thanks
Regards