You don't have to be a mechanic. You just have to understand the wisdom of the saying "Early bird gets the worm, but the second mouse gets the cheese."
Out of all the posts in the SLK section, I only see 2-3 posts about R171 (2005-present) and the rest of the complaints are about the R170 design (97-2004).
Sadly, MBZ is now cutting just about every corner they can to meet the price point and let the customers find out the hard way which shortcuts work and which ones don't. Then they quietly fix the top bugs in the later model years.
I used to own a 2003 MR2 Spyder, and the model years prior to 2003 had a common problem where the precat converter's ceramic lattice would start breaking up between 50k and 100k. It wouldn't be a big deal if it wasn't for the fact that the pieces would get sucked back into the engine and gradually scratch up the cylinders. Most owners only found it out when the car would lose half of its power and burn oil at the same rate as gas. Of course, the engine is gone by that point. 2003+ model years have a few tweaks to the manifold and the piping which is supposed to keep the ceramic bits out of the engine, but Toyota never officially admitted the problem.