Does somebody knows, if the following publication will be available (when?) as a searchable database, so that we beginners could quickly filter it for pieces of a certain difficulty level?

"Pianists Guide to Standard Teaching and Performance Literature"
Autor: Jane Magrath
ISBN 13: 9780882846552

There unfortunately is no such thing or plans in the works to make such a thing that I'm aware of. What you could do - that I've done with some success - in looking for material by "grade" (don't be overly reliant on this system, though) is use either one or some combination of the following resources in selecting pieces or composers out by grade(s) (difficulty), and then seeking out the material individually in the Magrath book for a better opinion:

http://www.gradedpianorepertoire.com/about/gpr_grading.php

https://www.box.com/shared/3dhioamzna
http://www.pianoworld.com/Uploads/files/Graded_Pieces_Sorted_By_Difficulty.PDF
www.pianostreet.com

I don't know, but I do highly recommed the book, one of the best purchases I have made.

I was actually already thinking about spending money on either the "gradedpianorepertoire.com" or the "pianostreet.com" database access. Does somebody has personal experience with these databases?
Pianostreet obviously has traffic in its forum, which is a good sign. Gradedpianorepertoire appears a kind of dead, I didn´t find movement on that site, which might be a sign that it is no more maintained?

On the latter yes. If you want to see the sheet music as well, it's very useful and most print sheets are nice quality. But you do not need to pay to just browse though the gradings and then get the sheet music elsewhere, like from the free databases or books.

Gradings differ. Magrath's book has it's own, which grades pieces that are within the scope of an intermadiate/early advanced learners. The grading is from 1 to 10 so it has more levels than most gradings I have seen. It also has explanations on the pieces, even some teaching notes. Pianostreet uses from 1 to 8+ for the whole repertoire, so it's a bit more rough.

You can also print many different exam system gradings lists free from the internet, like ABRSM.

I have used all these in combination actually...

I bought the Jane MaGrath book about 10 years ago and it is well-worn.

I have to credit this guide for getting me back into piano. About 30 years ago I despaired that I could ever play "well" and just gave it up. Silly. There is so-oo much good elementary and intermediate music for all tastes. Even if I'm never an advanced player I will not be able to play all the beautiful intermediate music in a lifetime.

It was worth the purchase for me!

I would only recommend pianostreet and even then, you're paying for public domain music that you could easily get through either a simple google search or imslp.org at a slightly lesser quality. Meanwhile, you can still search their database for free (for grades) and access all of the Beethoven sonatas, Bach two-part inventions, and others free with a free account.