This level is slightly difficult, as you need good timing to sedate the driver. And the game won't tell you where the Ort-Meyer keycard is. But follow these steps to get Silent Assassin and the keycard.
Sniping the targets in the restaurant
Run straight to the ladder which leads up to the second floor. Climb it and sneak to the first door on your right. Make sure that the guard does not see you. Pick the lock and take the bomb, remote, guard clothes and sniper from the room. Now go into the other door, then the front right door, then through the corridor, right past the elevator and through the next door into a balcony-like area. There you will have a clear view of two triad members inside the restaurant. Remember to dodge all guards, or your cover will be blown. Shoot them (you can get both with one shot). Now get back into the room which you initially entered by picking a lock. Drop the sniper and, if you want to, take your suit. Sneak back down the ladder (guard on balcony will blow your cover if he sees you).
Kill the remaining targets
Now cross the road and turn right into an area with lots of dumpsters. If timed correctly, the driver will be taking a leak there. Sneak up behind him and use the syringe. Now take the driver's clothes and drop his body into the sewer. WALK to the car (do not go too close to the guards, but then, you never should anyway :) ) and place the bomb. Now walk straight down the road and wait some distance away from the two triad members at the end of the road. A save is recommended when the remaining triad members get into the car. The car starts moving and when it is just about to exit, blow the bomb. The triad members inside as well as outside the car will be killed, and nobody will investigate. You may loot the Micro Uzis from them.
Get the Ort-Meyer keycard and escape
Before proceeding to your exit, you may want to pick up the Ort-Meyer keycard. Get back into the room where you left the sniper rifle. Once again, take the guard disguise and go the same way. Instead of going to the balcony, call the elevator just before that area. Go down to the basement. Cross the whole basement to another elevator. Remember not to run near any guards. Now take the elevator to the mansion. It will be dark and there will be no guards inside. You may need the night-vision goggles. Find the stairs to the second floor. Explore a bit and you will find a desk with a single light on somewhere in the room. The keycard is on the desk. Pick it up and retrace your steps all the way, to down the ladder. Go to your exit and you will get Silent Assassin rating. This works on all difficulties.
NOTE: You need to complete all missions up to Hunter and Hunted and play Asylum Aftermath in the completed game to be able to use the Ort-Meyer keycard.