I am planning on offering pen testing services as a freelancer. However, I am afraid that my customers may try to sue me for attacking their server if I don't have a legally binding contract stating that they comissioned me to. Therefore, I want to know what the simplest way to get such a contract without meeting in person would be (eg. are there any third party websites which are accepted as proof in a law suit?). Free alternatives are of course preferred.
If it makes any difference, I am stationed in Austria (-> Europe). Thanks in advance.