If you're seriously considering spending $5k on a warranty, may I suggest two alternatives:
1) Purchase a Starmark SL instead (they come with a basic warranty already).
2) Taking the $5k you would spend on the warranty and shifting the money to a newer, lower mileage vehicle.
Being in the same position 2 years ago, my E420 ate a transmission at $4,500 (replaced motor mounts at the same time). General maintenance and repair has run around $4k over the last 18 months. A good chunk of that $4k has been on the maintenance side. I don't know how much of the other $2k would have been covered under a warranty, but the transmission would have been covered.
My lesson in this story - alway go for lowest mileage and newest model year you can afford. At the end of day, the money probably balances out, but if you spared the hassle of going back & forth to your tech, etc., you'll enjoy the vehicle a lot more!