I read a different post about contact mics where people suggested that a good preamp is very important due to the high voltage needs of a contact microphone. So I was thinking of buying the Schertler Basik. Is this any good or is the price too high for such a mic? Also: this contact mic is meant for strings; do you think I can still use it properly for sound design purposes?
Can't comment on if that is a good mic or not, but I've started with self-made contact mics and would recommend those before spending hundreds of dollars. Building one doesn't require much skill and parts cost basically nothing. Soldering iron and basic soldering skills are plus, but you can try this even without by using electrical tape.
This is easy to follow tutorial: http://brokenpants.com/?page_id=94
As for preamp you need something with high impedance input. Your audio interface might have suitable one (usually labelled as Hi-Z or instrument input). Zoom H4 also works ok, since it's inputs are designed more for guitar than mics. Best solution would be some good DI box and good preamp, but I don't have enough experience with DI boxes to recommend one.
I have used contact mics for sound design in a live setting. I used cheap models off amazon that you would use for a pickup type of thing on a stringed instrument. All the ones I tried responded virtually the same way, so I won't get into brand, but I would definitely buy a couple in the 10-30$ range to test how they might work for your project. Some have built in clips which are handy, but for most applications just a clean puck-shaped thing is preferable. I ran them through ordinary mixer inputs and they did the trick. Their response was bass heavy especially as the impulse approached a direct hit on the mic, but I was able to filter it.
That is the option for if you don't feel like a DIY project, though Sauli is right too!
On the high end, something like a Barcus Berry Planar wave. The next step down is probably a JRF, which aren't very expensive. If your just looking to get some sounds captured in a different way or from a microphone that hears differently than cheep ones from Ebay like he_artburns mentioned are great or you can buy a pack of piezo discs for a couple bucks and solder them onto some cables. With those you don't really have to worry about putting them in harms way because they are so inexpensive. Like sauli mentioned, be sure to match them with an acceptable input, not a microphone input.
We make a low-cost contact mic specifically for sound design, with a built-in phantom-powered circuit that eliminates the need for the preamp. The Cortado buffers and balances the high impedance signal from the Piezo and matches it to the mic-in impedance of the console.