Hot water tank installation questions

[Austrian]

I finally got a replacement tank which you can see below. It has quite a few threaded holes that I won't use that will need sealing (their thread sizes shown in the diagram below). It also came with a thermometer that has to be installed in the small hole T. I have a few questions I hope someone can help with. Ideally, if there are help guides or videos on the web I can be pointed to (I've googled but clearly I'm not using the right terms as I haven't found anything useful).

• What's the best way to seal the threaded holes I won't use? Should I use tapered or parallel plugs with O rings (if with O rings, are there any particular types I should get)? And is brass or nickel plated best?
• How do I fit the thermometer? It goes in the hole T very loosely so some fitting must be needed for it.
• I intend to use flexible hoses so I can use the original tank's connections. I presume this won't be an issue.

 

Many thanks for any advice.