Just to clarify a couple things: the 07 "Bluetec's" doen't have the AdBlu (Urea) injection, that's only 2009+ models.
As for whether the om642 is a "bad" engine is entirely dependent on how you plan on having it serviced. are you going to do the work yourself? have an indy do it? or dealership? maintenance costs add up very quickly on these engines because they are quite complected compared to their gasoline counterparts. some people just cant live without the torque, however.
The om642 V6 is also a reliable engine, but it has a couple things i consider "barriers to entry" in reliability that need to be checked/fixed before i would consider the engine reliable.
-The oil cooler seals need to be done. You either need to have PHYSICAL PROOF (IE: bill) that the seals have been replaced in the past, or have them done. i own 2 om642 bluetec's, and worked on 2 others for friends; every single one of them had leaking oil cooler seals.
-Swirl flap system needs to be eliminated, either physically or electronically. there is a well documented trick to "delete" the swirl flaps using a single resistor in the swirl flap motor electrical connector that eliminates all check engine lights.
i would not consider a om642 vehicle in my opinion unless you don't mind spending what you paid for the vehicle in maintenance over a 5 year period at a independent Benz specialty shop or dealership, or unless you don't mind doing a majority of the repair/maintenance yourself. Contrary to popular belief, these cars are NOT terribly hard to work on, but there are jobs that are a lot more frustrating than others (Oil cooler seals).
The om642 is a great engine in my opinion, as long as they're actually maintained properly.