Have a scoot around on google, if you also search 'SLK320' there is likely to be more on the Mercedes forums about any potential issues, simply because there are more SLK320's around (same engine, gearbox and chassis)

Generally these cars are very reliable, and I dont think engine mounts are a common problem, so you might just have something loose somewhere, or something as simple as a sloppy gear linkage, but its worth investigating.

Mine has done 66000 miles and the gearchange is still slick and clean, the clutch however (as previously mentioned) is definately an aquired taste which takes a couple of days to get used to, I thought it was just me at first but i've let 3 or 4 people drive mine since i've had it, and every one of them stalled it a couple of times.