Unless you can do it crazy quick + cheap, try to avoid building product until you have cleared your MVT.
Personally, I would spend 5000 euro on marketing or marketing time-equivalent efforts trying to validate the concept before spending even $100 on the product.
You don’t need a minimum viable product (MVP) — you need a minimum viable test (MVT).
Check out this article from First Round for overview on MVTs. https://review.firstround.com/the-minimum-viable-testing-process-for-evaluating-startup-ideas
Unless you can do it crazy quick + cheap, try to avoid building product until you have cleared your MVT.
Personally, I would spend 5000 euro on marketing or marketing time-equivalent efforts trying to validate the concept before spending even $100 on the product.