as I'm preparing for 1kyuu and laying my plans for the future , I'm currently wondering about learning chinese earlier than I planned on the beginning.
which kanji are common to chinese and japanese (writing)?
Even if I cannot fathom how much work will be needed before actually coping with it , I'd like to know if you know a website which list them or if you have a list of your own. Kanjis are numberable so it would give me something to chew on before coming up with a real battle plan : I know that anyway there's still reading for every and each single kanji + grammar.... Still it would save me some sweat .....

Um - most of them. Unless you're talking current Mandarin, in which case you'll see a bunch of simplified kanji. With Taiwanese, many modern Japanese kanji have been simplified as compared to the older Chinese forms, but of course not in the same ways as Mandarin characters.

You might want to clarify what you mean and which direction you're thinking of going.

Heisig RtK1 has all of the jouyou kanji and while that might not be all-encompassing, it's as close as you're going to get I'd say...

For Chinese, I'm guessing Remembering the Hanzi is built in the same way, aiming for the bare minimum for fluency.