I would check out Kelley Blue Book on this car and see what the value is:
www.kbb.com
I would also run a carfax on it to see if it has an accident history.
Last, it is always a good idea to take it to the dealer or a respected MB mechanic and get a PPI (pre-purchase inspection) done. It may cost $100 or so, but they're worth it.
I also think it is the general consensus on this forum that it's always a good idea to get the longest extended warranty you can purchase. There are just so many expensive components that can go on cars these days, that it's good insurance to have. You mention it is "Certified". If this is a Starmark warranty car, then it has a fabulous warranty. I believe these can be extended, and if it can be extended, I'd do it.
Having said that, the E320's are great cars, and there are really no weak spots to look out for.