Lagers typically do not need all that blowoff, as they don't ferment as quickly or "violently" as ales. The lower temperature helps. However, like Gila says, be prepared. Yeast doesn't ask your permission before doing whatever it wants to do.
I recommend against crashing, though. When you go down to lagering temperatures, you do so gently. Maybe drop about five degrees (F) per day over a few days. I usually drop the temp three degrees (F) in the morning, and then two more at night. You don't want the yeast to just shut down. You need it to properly lager your beer.
Lagering in the keg is fine. With the time and temperature for lagering, your beer should clear out well, and the yeast will pack down well at the bottom. Yes, go ahead and purge your keg. Go ahead and hook it up to the CO2, as well, if you want. It won't mess anything up.