What is the best plugin for a Bilingual Site with these features?

Look here what plugins are available: http://codex.wordpress.org/Multilingual_WordPress

I don’t want a multi-site installation. And I can’t afford to pay for WMPL.

Maybe one of these:
http://wordpress.org/extend/plugins/polylang/
http://wordpress.org/extend/plugins/xili-language/

both language versions of the post should have the same comments.

I don’t think this is possible with any of the plugins, but I’m not sure.

Both plugins polylang and xili-language will do all the rest of your requirements.

http://codex.wordpress.org/Multilingual_WordPress#How_to_choose_the_right_multilingual_solution

In any case, installing a multilingual plugin is a big change for any site. It would be a good idea to first create a test site and verify that everything works correctly between all the required plugins and the theme and only then install. Since any multilingual plugin changes the database significantly, doing a database backup is required before experimenting.