Generally very reliable if they are serviced properly, but a few known things too look out for.
Make sure the gearbox has been serviced, fluid and filter, every 4 years or about 40 k miles ( whichever soonest ) If it has FMBSH it should have been done unless previous owners decided to save money!
If hasn't been done the gear changes when box warmed up may be less smooth, should be seamless , but in any case even if it drives well budget for a Fluid and filter change immediately approx £200 at MB Indi ,.
Quality of servicing the key , much better if done at MB dealers or known MB Indie's, I would be more wary if at unknown garages, especially if it looked and drove anything less than perfectly and had more than 2/3 owners. Take a good look at the MOT history using the online checker to see fails / advisories.
Motors can have swirl flap failures or leaks around the injectors ( commonly known as the Black Death )
I had a 2008 E320cdi and loved it , only had an ABS wheel sensor fail, not too costly £200 approx.
Only sold it to buy a ML 320cdi. If you like it go for it.