Don't get your wife the 300D, get her the 240D. It's simpler to work on, the a/c, heater always works. Get the 4 speed manual transmission, it never fails and you know when it's going to shift as long as your leg works. The windows are manual and so the motors never fail. The steering is quicker and the spring rate is stiffer for winding roads.
They handle highway speeds ok up to seventy when they get a bit noisy. If you're really lucky, find her one of the euro 5 speeds that come up evey so often. At highway speeds above seventy, they're really quite. Then, if you so desire later, you can stuff it with a 300d engine or even a 300DT engine. Then the fun starts.
For an all round commuter you can't beat a 240D and you'll love how easy they are to work on. BTW, they're a lot less demanding mechanically than a 280!