May I personally use CDN for whole website (PHP, Apache, MySQL) or simply for images and CSS, JS files?

What's the best option > cloud-hosting or devoted-hosting? Does CDN has that support?

Witch hosting you suggest me the very best - the quickest, stablest 100% uptime, CDN, not costly whatsoever?

From how a question was phrased, I believe handled hosting could be most suitable for the application.

It's fairly unlikely that you'll encounter any performance problems that are not self-caused (say, by writing suboptimal database queries, carrying out database processing within the frontend etc) unless of course you've got a significant advertising budget along with a store bought application, by which situation it's also wise to possess a mid-sized IT department that may roll a custom solution.

Weighing cost and reliability against one another could be left towards the an accounting firm, typically.