The WK2 is not going to use the Mercedes OM642 V6, but a new 3.0 Litre V6 by VM Motori and FPT Powertrain. According to http://www.vmmotori.it/en/01/00/01/dettaglio.jsp?id=9 the new engine develops 250 HP (184 Kw) and 404 lb/ft (550 Nm)
Jeep UK have just released a teaser video, as did the US website before the release of the Grand Cherokee WK2. This one however shows the Diesel engine. I read an article in Auto Express magazine which stated that the Diesel will have a six speed gearbox when it debuts in June.