The oil filter is in the back of the engine close to the firewall. It is somewhat buried. You can buy a "socket" from the dealer that will help removing it.
I only use Mercedes oil filters. The reason for this is actually from an old Toyota experience- I learned that Toyota oil filters have an integrated check valve which prevents oil from leaking down while the engine sits, not running. When the engine is first started there is almost instant oil pressure.
Now I don't know if this is true with Mercedes, I have never heard of this, but Mercedes has been known to change and update their filters and recommendations over the years. They obviously care about the longevity of their vehicles.
I have tried other filters but have kept coming back to the Mercedes ones. In a nutshell, I trust their filters.
To drain the oil, yes there is a drain plug a the very bottom of your engine. It will allow for almost a complete oil drain, which is good. You will notice that the drain plug has an washer. It is called a crush washer. It is to be used, new each time.
Coincidentally, your new Mercedes oil filter often comes with this washer.
Hope this helps,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)