my question is what log books do these aircraft have to legally carry, how accurate will these be bearing in mind the self-maintenance aspect of these machines and how familiar are engineers with the types.
If we are talking about an LAA permit aircraft then they have to have log books which are very similar to certified aircraft. The range of pilot maintenance allowed is also very similar but in the case of the permit aircraft any other work has to be signed off by an LAA inspector.
You would be advised to take along an LAA inspector, familiar with the type, for a pre purchase inspection in the same way you would take a licensed engineer if you were buying a C of A aircraft. Indeed many LAA inspectors are also licensed engineers.
I notice the PilotDar is from Canada and it is worth remembering that the experimental category there is very different from the system we have to operate under in the UK.