I passed the N1 several years ago mostly using the 日本語総まとめ series and some other resources. I would say both that series and the 新完全マスター series have their own benefits and problems. The 日本語総まとめ books have some typos and a few of the quizzes list the wrong correct answer, but the main issue is that the translations are generally poor, if not outright wrong. If you ignore the provided translations and just rely on the Japanese text, I think the books are still pretty useful, and I found that the quizzes and tests included were more helpful in preparing for the actual test than the ones in the 新完全マスター books. I would say that the 新完全マスター books are more professionally done, but you will definitely end up being overprepared if you study all the material in them. You can see that as a good or bad thing, I'll just say that the actual test material when I took it was a lot easier than the stuff in the 新完全マスター books. Also, one cautionary note: the 新完全マスター books are more professionally done, so you may feel that means the material is more trustworthy, but that's not necessarily the case; it still has occasional errors, mainly because it seems to take a prescriptivist approach to grammar.
I would say doing a lot of practice exercises modeled after the actual test helped me a lot, since a lot of people trip up over the format if they aren't used to it. You can find some materials online, but there are also books you can buy with just practice quizzes and tests in them, including several supplements to the 日本語総まとめ books. Doing a lot of reading and listening practice with native materials was also key; I believe I was one of the only people in my test group to actually finish the entire reading section (with time to spare!), which I owe to reading both light novels and non-fiction books without using a dictionary. Reading without the help of reference materials will improve your reading speed, and also improve your ability to read around the parts you don't understand, a skill I think a lot of people lack. And I think it's important that you don't only read fiction, since the N1 level material covers a lot of formal grammar patterns and compound nouns that you're not going to find in most novels, and on the actual test, the reading section will contain a lot of essay-like material, rather than novel excerpts; fiction is good for improving your vocab and grammar skills, though. For listening practice, I watched a lot of anime I hadn't seen before without subtitles, and played Japanese videogames with extensive voice acting. Finally, in the month or so leading up to the test, I crammed vocab on renshuu, putting all the N1 vocab in a schedule, then studying all available terms each day. This was before there were grammar questions on renshuu, but I imagine studying those would be helpful as well.
As far as sources, I've used White Rabbit Japan and Amazon Japan to order a lot of my books, you may be able to get stuff from there if they ship to your country. I also get some of my reading material from my local Bookoff and Kinokuniya stores, not sure if they have those or anything similar in Europe.
