Ive had one for a little over a month, it should drive like a e320 with a LOT more torque.
Make sure the engine has no rattles, Ive driven a couple which had this problem. If the speedo flickkers its not the simple cable or speedo head fix, the amg has a gear set to adapt the speedo, circa £500. Check the usual M104 issues.
From memory he wanted around the £5k mark for it last time, which in my opinion is overpriced for w124 saloon. He may have come down on that.