I don't have a domain with them. I use Namecheap. But I have heard from colleagues that use Netcup for their domains that it is quite cheap for some tlds.

I currently have 2 Root Servers and 1 Storage Server with Netcup for quite a while now. I haven't had any issues so far. RS 1000, RS 2000 and S 1000.

Now for your questions.

Yes you can use Let's Encrypt with the server.

As far as I know you can set up a mail server on there. Maybe write the support team or ask in the forum if you wanna be 100% sure it works.

That should work as you have a satisfaction guarantee for the root server. Support will probably be able to change if for you.

I run Matrix/Element on my RS 1000, and it is more than I need. Only uses about 4 GB of RAM at most. On the RS 2000 I currently run 44 Docker containers with no problem.

But to come back to your question, if it is worth saving the buck, then for your use case of 1 or 2 containers, it will probably be fine.

You can check the price on the site. If you already have one, it depends on whether you want to transfer it to them, so you can manage everything in one place.