The MB W123 makes an excellent daily driver.
MB sold over 5 million W123s world wide during its production run from 1977 to 1985. A few were imported in 1986 as station wagons only.
MB 240Ds were imported from 1977 to 1983. In 1984 the new and smaller W201 body 190D replaced the 240D as the bottom diesel model. A mint condition 83 240D under 100,000 miles should sell for no more than $5,000. Typical prices are $1,000 to $3,000, depending on mileage and condition.
The 240D will be as fast/slow as a non-turbo VW 1.6 engine. Mileage will be around 30 MPG give or take a few either way, depending on the state of tune. The difference will be that a 240D is a large well made car compared to any VW, especially considering the age. A W123 weighs in at 3,500 lbs. and has about the same interior space as a Passat.
The W123 body is mechanically straight forward to work on. The cars are simply over engineered and built and are not complicated. Parts are easy to get and reasonable, even from MB. I can get parts for my MB cheaper than my VW. If you have the skills/tools to work on a VW, then you can work on an old MB.
Tell your friend not to pass up any good 300Ds or 300D-Ts (T for turbos). That extra HP really comes in handy and mileage is only slightly lower. The 300D non-turbo was imported from 1977 to 1981. All 300Ds from 1982 to 1985 are turbocharged. 300Ds/D-Ts sell anywhere from $1,500 to 10,000. Low mileage coupes and wagons are on the high side. Typical prices for 300Ds runs from $2,000 to $6,000.
All the MB diesel engines used in the 240D/300D models are almost unbreakable. The only way to kill one is to neglect the oil and/or timing chain. The timing chain and tensioner should be replaced by 150,000 miles. Replacing the timing chain is easier and cheaper than replacing the timing belt on a VW TDI engine.
Besides oil & filter changes, the only routine maintenance item is having the valves adjusted every 15,000 miles. A new valve cover gasket is installed with every valve adjustment. MB makes an excellent auto-tranny. If the tranny fluid & filter was changed every 50,000 miles, the trannies last forever.
As with any car over 15 years old, things wear out. HVAC problems can be expensive to fix. The rubber around the glass, body and suspension bushings will be cracked and dry rotted. Vacuum leaks are common, which effects EVERYTHING. These items are usually just tedious to find and fix, not impossible. Most MB owners keep good records of all repairs and routine maintenace.
A good place to get introduced to MBs is at www.mbz.org/.
From there you can get links to just about anything MB.
Brian, 97 Passat TDI, 85 MB 300D-T