While this is a good site for this type of information, you may also want to review the archives at www.mbcoupes.com
This site is heavily into stock and modified w126 coupes.
Your budget sounds more like a 380 or 500 SEC than a 560. Watch the pre-'84 380's due to the weak single-row timing chain. A conversion kit is available, but will put a dent into your budget.