Yes, like Enga said, you can change it from the top, behind the engine block.
Yes, head gaskets were a common problem on the mid 90s M104 engine (280 and 320), make sure it has been replaced, or atleast get an extended warranty. That itself is over a grand to fix. The wiring harness is about a grand, I believe. Make sure it's in good condition. Gas mileage is the same, about 26-27 on the hwy, driving 'conservatively.' And the city I get about 20 mpg when driving the same. Even when I drive it hard, with a mix of city/hwy driving, I still get ATLEAST 20 mpg, and that's with HARD driving.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)