Please recommend a CD/tuner up front which will suit a SLK.

Reckon Blaupunkt Los Angeles will suit it nicely as it is also made in Germany! Other than that the following are my recommendations:

Rockford Fosgate RF5.3x -2 channels
Cerwin Vega Stealth S-8s woofers
Alpine Bass Shakers (to be placed under the seats)
More body deadener for resonance stopping

Disadvantage of a convertible is that you would receive less bass gain with a 12" sub compared with a 10" sub in an enclosed car, example a Civic. So you must have EXTRA POWER for the higher ambient noise level especially when the roof is down.
 

Which is a smarter purchase with regards to sound acoustics and stereo installations, the Mitsubishi Lancer GSR sedan or a Toyota Celica GT-Four Hatch?

I am a great fan for both cars. But neither car offer outstanding fornt stage mounting locations if you are really in for top notch stereo imaging. Even if you are willing to put in ALOT, ALOT of money to fund major works like laser cutting body panels for serious modifications, I don't think either of these cars lend themselves well to being full-on competition Sound-off cars. They can only be made to sound good for everyday use. *sigh*

So make you choice on which car is better to drive or just simply flip a coin. That always works!