Multisite issue

Hello

I would like to develop a multisite (e.g. example.com) with subdomains like this:
1) test.example.com - that would be for testing purposes only
2) en.example.com - that would be the main site in english language (user interface and content should be displayed in english)
3) de.example.com - that would be the main site in german language (user interface and content should be displayed in german)
4) el.example.com - that would be the main site in greek language (user interface and content should be displayed in greek)
5) and so on... other subdomains might be adder later for specific purposes
I have read some other posts. And I would prefer (if possible) to use only CORE modules. I would also like to hit example.com and be automatically redireced to el.example.com for example (browser detection). But (this might be hard!) I would also like to login only ONCE to all subdomains. And of course, if a user understands another language to be able to change and be directed to de.example.com for example. The way I see it, I need the folowing:
1) 4 or more drupal installations, as many as the subdomains.
2) ONE common database, since I should share some tables.
3) DIFFERENT prefixes eg test, en, de, el and so on for most of the tables
4) A common prefix (or NO prefix) for which tables??? only for users and users_roles tables???
5) 4 or more sumbolic links, I know how to do that...
6) Some sort of trick for the frontpage, for the automatic redirection, etc but what's best???
7) Go to each subdomain and adjust the language (using locale module)
8) Anything else???
What is necessary for me is some advice from experienced (in multisites) drupalers. To help me see if my approach is good and also direct me to do the EXACT steps the right way.

Thank you in advance