As ever, an unhappy customer does not necessarily mean the supplier (or customer!) was at fault.
When employing a surveyor, you do need to agree what depth of inspection you want; the more you want checked, the more the time and cost. The link refers to an "engine survey which cost €300". Wiring would not necessarily be a part of 'an engine survey'. You certainly don't get an in-depth survey of a boat for €300.
Coers are certainly a qualified and well-established company (http://coersexpertise.nl/language/english/index.html).
Pete