* Buy-back would be voluntary, cut Greek debt by 11 percent of GDP * Lawyers preparing briefing for euro zone meeting * Germany, others, support cutting interest on bilateral loans * Germany, others support raising Greek primary surplus target to 5.0 pct/GDP from 4.5 pct in 2016 By Jan Strupczewski BERLIN, Nov 19 (Reuters) - Germany wants Greece to buy back half of its outstanding bonds from private investors at 25 percent of their value as one way to reduce its unsustainable debt, a source familiar with preparations for this week's euro zone talks said on Monday. The voluntary proposal would leave private sector holders of Greek debt who have already seen most of their investments wiped out...