Old C-Class: Well, we don't need to talk about this! Just awful! Looks more like a Premium Economy. Bad seats, small video screens, etc.

New C-Class: Quite good. Not completely full-flat, but angled-flat. Huge screens (larger than F-Class). You can sleep easily. Besides, this seat offers many small features like space for your glasses, shoes, laptop, paper, etc.

I've just one hint for you: Try to choose clothes which are not too smooth/slippy. Otherwise you will feel the consequences of the angled seat as you will slip down while trying to sleep. That was my experiencce when I took LH's new C-Class for the first time.

Food: Okay. Not as good as Singapore Airlines for example, but still okay. You'll get three courses plus coffee, ...
Before landing, they will either offer a small dinner or another complete meal, I'm not sure. On the return, normal three-course-dinner plus breakfast in the morning. You can usually choose between omelette or a cold plate.

Lounges: Don't expect too much of the Lufthansa Business Lounges in Frankfurt. Some drinks, snacks and newspapers, that's it.

Why do you not take the A330 on the return, instead of the A340. If you are flexible, you can change to another day of the week and enjoy new C-Class with the A330.