Which kotatsus have a good quality and are cheap (max. Approx. 400, - €)
HMMM, difficult. Due to the more low demand here, the things are quite expensive. How about a home?
preferably in Rakuten directly from Japan. The deliver to Germany.
But a Kotatsu pulls a lot of electricity, and there is no primitive plug adapter!
At Conrad in Germany you can buy a sufficiently powerful adapter transformer. Costs about 50 to 70 euros in addition, but is highly recommended if you do not want you to flip the booth.