Do you know where one can find drm-lite versions of Dungeons & Dragons core rulebooks? Such as the Dungeon Master's guide, Player's handbook, Monster manual?

It seems the three would cost about 100€ / 130$ in printed versions, and I can't find them online as pdf. drivethrurpg.com sells their supplements but does not offer the basic tools...

You could take a look at some of the rule-sets in the form of an SRD or rules restatement.

Consider the following:

SoveliorSage

OSRIC

Probably in the back of my closet or in the attic of my parent's house.

Definitely my closet, I still have the red book and blue book of original D&D

About halfway through the life of 4th Edition I think they streamlined the products some more and in turn made it more confusing. It's possible what you need is the Dungeon Master's Kit.
I think they even did away with the Player's Handbooks, instead forcing you to use their Character Builder.
I'm not 100% sure, but I think that's what happened after the release of 'The Red Box' and such.
That's assuming you want 4th Edition of course (I think 5th is coming soon).

You might also want to consider Pathfinder which is partly based on the 3.5 rules.

The real question is what edition of D&D you are speaking of. As to online tools, there is always Hero Lab.
http://www.wolflair.com/index.php?context=hero_lab

It's regularly updated. You have to pay extra for supplements, but it is licensed. Works great for Pathfinder which some call D&D 3.75. I haven't used the 4th Edition tools, but it probably works well for that too.

The 3.5 edition material is a little hard to get your hands on these days. Due to a reprinting a few years back the core rulebooks can still be found new, but the other supplements are quite challenging to get your hands on. Your best course of action is to go hunting on Amazon if you want physical copies. I'm pretty sure that if you want PDF's from legit sources, you're stuck with whatever DRM scheme WotC is using. You can find most of the core rules online anyways so I'm not sure if there's much point to a PDF of the core rules. Another option you may want to look at is Pathfinder. This is essentially a third party re-release of the 3rd edition rules which gets more up-to-date support. Almost all of the rules are online so, again, I'm not seeing much need for a PDF copy there. All the Pathfinder rulebooks are still in print, so buying them new is pretty easy.

As RWarehall said in the post above mine, which edition of the rules are you interested in? The cost and availability can vary wildly. If it's 3rd Edition you crave, Pathfinder is tops and it's a single rulebook purchase (save for the monster books).

If it's older editions, there are a lot of good retroclones out there like Labyrinth Lord or OSRIC (which Pseudoman recommended already).

Technically you can also use one of the many applications for offline browsing to store and view websites and such and even turn them into PDFs or you can download PDF versions of the SRD [url=http://www.systemreferencedocuments.org/index.php?option=com_content&view=article&id=130:dad-35]from websites[/url].
But I prefer something that's also fun to look at with nice artwork and such,

Library!

The Pathfinder pdf is, quite honestly, glorious, and price-wise, it's a great buy for the content you receive. I would never try to print anything other than a page or two (my ink! My precious ink!) but if you have a tablet it's very, very handy to have at the gaming table. Especially since the giant behemoth of a book tends to get worn out easily.

That said, I don't know how convenient Paizo's webstore is to non-US residents, so I couldn't speak to that.

But if it works, get the core pdf and the first monster book and bingo! You're good to go for quite a while. In fact, I spent nearly a year when I was between jobs with just those two books and some freebie adventures as my primary roleplaying collection.

http://www.dndclassics.com/

I've already checked that one, it's a sister site to drivethrurpg, and so it has the same limitations.

Crikey, the Gog Forum Codex does not mention how to share XP equally at the end of a scenario, so I up-repped the entire party. Kudos to you all! I now have viable options for a bright roleplaying future!

A quick Google Search gleaned:

Player Handbook 3.5 on .pdf

4th Edition Core Rules .pdf

Most 3.5 Book as .pdf