1. 21% of your fee goes behind taxes.
2. If you want to work on your own you have to pay each month around 250€
3. If you are going to live there, you should also consider paying the social security fees (around 1000€/month more).
More input: A house keeper charges between 10/12€/hour,
A particular teacher charges between 15€/hour.
A computer forensic expert charges around 100-150€/hour.
Mechanics charge between 30-50€/hour.
I would recommend between 75-150€/hour depending on the amount of hours. If you are going to stay in their office and work with them around 8h/month I would charge between 5-7k/month. 1k/week should be fine.