Hi,
When you setup this with CDN + External storage you are only leveraging traffic, which indeed helps a lot to make your website more reliable and faster, but if your website will get let say 1000 uploads per minute then the machine won't be able to process that without delays, and I'm talking about any shared server out there.
In this case I think that the best for you is start with shared hosting + external storage (like S3) and then upgrade the hosting plan to either VPS or dedicated when you noticed that you need to make that move. Key element is the external storage, so if your website grows fast then the migration is quite easy because the files will keep in the same external server, you will only move the database and the core files.