Hello Schuckie,
the property will not become 12% more expensive in 1.5 years, unless something big happens locally. So get the current standard land value map and compare it with 2013, but the value is likely to be the same.
Based solely on your descriptions without a local inspection, the price cannot be assessed, but I am a bit bothered by your statement that everything was built simply - because then the price in my opinion does not fit, but would have to be set lower. However, this can be justified due to the (market) situation.
Roughly from a distance and keeping "simple construction" in mind, I would estimate it at around 330-350K€, based on your area details, i.e., as a buyer I would try to get it for 340K€, or the location and room layout would have to be very advantageous for me.
Your current final price including incidental costs (12%) and garden/painting work etc. (20K€) amounts to approximately 445K€ - if you buy for 340K€, the final all-inclusive price would be around 400K€ ... just a thought.
During the inspection with AR, he will also assess the self-performed work, as doc.schnaggl also describes. It can be well done, but that is rather rare and therefore a point where the price can be adjusted.
Best regards Dirk Grafe