Earlier quoted context omitted.
This also means that you have to invest 200€ in addition to the time spent coding it and to the hosting+domain fees in order to get things started. Considering the goal you expressed (making passive money), why was this your choice compared to spend less money at first, code it and just make it functional (not beautiful), and only if it works or show signs that it will, then spend the 200€ (or even more then) on the…
Apart from having a landing page and trying to get it ranked for certain terms, I don't think you can validate an idea without being able to show/tell people what it will look like. The quality of the product will be judged upon what it looks like, if I made a 1995's style white page with just two form fields and a 'submit' button, that couldn't be used to validate the idea with. In my mostly-unqualified-to-comment-o…
I decided to spend a minimum amount of time on it to just test it out: just a form and a little description. The only decoration was that the text was centered and navy :)
Results: only 4 sign ups over 3 months..mostly from friends and family.
Then I made a nicer design, added content, and voila got about 30 sign ups in about 1 month through google search results. Added benefit is that now I'm not embarrassed to promote the site which of course has yielded many more sign ups.
To sum up: an effort invested in design pays off since good looks provide a hint of legitimacy. Generally speaking :)