There is not really a rule of thumb on mileage when it comes to MERCEDES-BENZ motorcars. Always remember durability and long service life is what sets these cars apart from other marques. Like the others have said, look for service records because if worst comes to worst you will just have to perform services that should have been done. My last MBZ I bought had 175K miles but all major services had already been done. Now 50,000 miles later the only things I have had to do apart from routine maint. were to replace the battery and alternator and buy new tires. Oh yea, I had to recharge the A/C big deal. This car was a gem!
Bottom line--- Don't be put off by E320's with over 120K miles, I have seen many older M103 cars with 300K plus at the dealership sevice dept.
89'300e
92'400e
92'500e