The barn conversion: a popular and complicated topic in rural areas.
First, the construction and planning law situation must be examined and the question whether a conversion for residential purposes is even approvable. If the barn is located in the outer area (§ 35 Building Code), the conversion may possibly not be approved. In addition, it is a so-called "change of use" which requires a building application. If the barn is adjacent to the boundary, questions of fire protection (building boundary wall) and residential lighting play a role. Possibly, distance areas to the neighbor will be triggered. This should be clarified in a preliminary building inquiry. Is there monument protection? The barn must also be accessible via a secured access and of course be connected to the usual supply and disposal lines.
The briefly described wall construction very likely does not even come close to meeting the requirements of the Energy Saving Ordinance for building insulation. This also includes roof insulation/insulation of the top floor ceiling and all floor surfaces against earth or above a basement.
I recommend that you, together with a planner experienced in old buildings (civil engineer/architect), carry out a building survey, determine the condition and possibly identify building damages/moisture damages. Old agricultural buildings are also usually not sufficiently/frost-free founded; this causes the structural engineer for the change of use with new building loads quite a headache. A renovation and conversion concept should then be created in connection with the entire construction project, which also takes into account the construction and planning law issues. For the preliminary building inquiry and the building application, you must engage a planner authorized to submit plans.