With my experience you can start without any issue with a CPX11 my only worry is the Disk Space as you will fill it pretty quickly. Like OOO you’ll have a 65-70% disk usage, so just to avoid soon off line time for upgrade I’d go with CPX21 but not because you need the Horse Power in the server.
Other option is to host all the uploads in a S3 or S3 like offering, the thing is that I’m almost sure that Hetzner doesn’t offer this product yet. Using S3 would allow you to offload all the attachments to another hosting which is a lot cheaper.
The CDN is a Content Delivery Network this service will cache your most used static assets as JS, images, etc. and distributed in multiple Point of Presence (PoPs) which lower the load on your server and increase the load speed if your community is spread in multiple regions.
If you want you can send me a PM and can give you some more advice.