Hi Jack,
Reliability should be excellent, as long as service history is maintained
(as this previous model, 476,500 mile example shows) - though I suspect it won't have any because of the age of the car you're looking for. Service intervals are usually yearly or about 15,000 miles (depending on usage), whichever comes first.
If you're going to keep the car for a while, ensure the gearbox oil and filter is changed and completely flushed about every 40,000 miles or so. Mercedes says it's a sealed for life unit -it isn't.
Main dealer prices are extortionate, in my opinion. I have to get mine done there, as it's part of the leasing agreement, and the first service on mine was almost £280. A good independent/specialist, will be about £180 for a minor service, £280 for a major (and this is for the 6-cylinder E320 CDi) - and will do a proper job on the gearbox as mentioned. Where I had it done at a Mercedes dealer, I believe they just topped up the oil. When the independent had a look at it and did the oil change - it came out black.
However, you may wish to keep to main dealer servicing if you don't plan to keep your car for more than a few years. If not, then independent/specialist servicing won't see the car's value drop too much (against a full dealer service history), as by then, the condition will affect the car's value more. Nevertheless keeping main dealer servicing, renews Mobilo (breakdown service) until the next service.
Road tax should be £115 a year, and MPG wise expect low 50s on motorways, mid-to high 30s in town.