The internal condition of any diesel is appraised by a compression test. So make that part of any purchase agreement; it costs a bit but will avoid a lot of expense afterward.
I've owned a '80 300SD M-B diesel since it was new and have a few opinions about diesels. The old, all iron motors (thru '85) are tough, durable engines that can take a lot of use and abuse. Old, all mechanical technology that lends itself to the DIY owner. The later engines have better performance but are not as tough as these old dogs.
Yes, some engines have lasted for vast mileages but those were fastidiously maintained, perhaps over maintained. IMHO the useful life of a reasonably maintained all iron diesel is about 275K miles +/- 10% after which it will need an overhaul, probably the transmission as well. The newer motors' iron blocks and aluminum cylinder heads may need a head gasket before they reach an overhaul.
As to fuel economy, the old 300SD consistently delivers 24 mpg in commute driving. Notice the diesels' relatively high city mileage for it's their main benefit, a reasonably sized gas engine will approach diesel mileage on the open road. My '97 E320 which has much better performance achieves and 26 - 28 mpg on the road, about the same as the old diesel.
M-B will be selling a new E320 V-6 diesel starting in October. This motor has better acceleration than their 3.5L V-6 gas engine and the diesel's mileage is about 33% better than the gas motor! Both of these motors are "state of the art" producing 75 HP/L and I relate this so you see what can be achieved. But this new diesel is more complicated than its V-6 gas sibling and that forsakes the diesel's traditional simplicity. I'm thinking about buying one but am quite wary of its complexity.
You should know that the diesels require more frequent oil and filter changes than do the gas motors - it varies by engine so check that before you buy. Diesel fuel now costs more than premium gas (San Francisco Bay Area) so the fuel isn't "cheap" and it will head higher when the sulfur is removed this summer.
A diesel helps but it isn't the magic solution to high fuel costs; that can only come from lighter cars and smaller engines.
The MBUSA site has a menu of "prior models" that may help your investigation.