"I'm guessing the 4-speed 4-banger would get better fuel economy than the car I was thinking of upgrading to, a 300D turbo "

Sorry, I do not think that is the case.... The turbo engine loves being forcefed... I got 27 mpg at 75 on a 280,000 mile 81 wagon... while my non turbo 240d gets about 24....
The ideal combo... turbo and four or five speed manual with ' correct' rear end ratio.