There are multiple offers that will ship to germany for ~11-12 EUR shipping costs on top of the 8-15 EUR for a device with OEM firmware (>>10 listed offers, more if you include the two identical variants) or 25-30 EUR for ones preflashed with OpenWrt (~5 listed offers). Include ~5 EUR for a UK style power adaptor into your calculation.
I wouldn't wait until late march though.
Yeah, is very common in the UK and there are lots in the UK.
As @slh said, make sure it's Type A. It's on a sticker at the bottom of the router, do if the seller don't mention the type in their ad, ask them to confirm. Or maybe better to confirm our send you a photo of the sticker anyway, because some sellers might not know the are types! So better be sure than get it and then it's Tyoe B which you can't flash.
Thank you for your answers.
Since I only need the 5 GHz band and I really like the quality of the AVM products, it will probably be a Fritz!Box 3370.
Is there something I should pay attention to? For example, a new or old revision? Hardware that is different, or something else?
In summary, VDSL with 50Mbitps (in Germany) and 5 GHz work with OpenWrt 18.06 on this device?
The commit liner notes say only hardware revisions 2 and up are supported. I am actually not sure if a "hardware revision 1" ever existed, but there's an easy way to get a later production model. The 3370 has been produced between 2010 and something like 2013, 2014. If you look at the serial number, and most sellers conveniently show it in their auctions, it starts with a letter. A stands for 2010, B for 2011, and so on, so I would choose one that starts with a C. That should get you conveniently into "not revision 1" territory.
As for the basic hardware, the 3370 is not any more or less capable than a Home Box 5A, or a TP-Link W9980, they all use the same SoC. I use a W9980 on a 100/40 line (also in Germany) without any problems, but I also think that it's pretty much the limit of what the chipset can handle. You should be fine with a 50 mbit line, and for anything that goes way beyond something like a 100 mbit line you'd have to look into completely different hardware anyway.
This is a good hint.
Can you explain me what this means:
Since commit 0b62fe5e OpenWrt can be installed using the EVA/Adam2 bootloader. Within the same HW revision 5 of the FRITZ!Box 3370 different NAND flash chips are used and the correct image has to be chosen. Please read the detailed commit message before trying an install.
This means that I should open the box, check wich NAND flash is used an choose the right image??
Exactly. I do not know how if hardware versions other than 5 all have the same flash chip, and if so which one that would be. I would open it anway just to make sure.
Also, I just noticed that this and other 3370-related commits never made it into 18.06, so it would be a good idea to go straight to master snapshots.
So I've ordered a 3370 now. 
Serial number begins with E264.XXX so the chances are good.
so it would be a good idea to go straight to master snapshots.
Could you put a link what you mean?
I hope that I will not brick the device... with a 6490 (KDG) I already had to make negative experiences. 
Thank you.
I'm a bit confused. Why a snapshot and no stable 18.06.2 like on the openwrt page? I noticed that there is a 18.06.2 build, but it seems only for sysupgrade? No kernel etc.
https://openwrt.org/toh/avm/fritz.box.wlan.3370
I'm really not a fan of unstable software on a server (due to bad experiences).
https://openwrt.org/toh/avm/fritz.box.wlan.3370
Appears you need to do initial install with snapshot and then you can upgrade to release from that.
The reason is:
Does anybody know if I can upgrade from the snapshot to the stable build? @takimata which version do you run, snapshot/stable?
And why if "3370-related commits never made it into 18.06" on the openwrt page current release is 18.06.2 and in the v18.01.0-rc1 Changelog: complete AVM FRITZ!Box 3370 support ?
Very confusing.
Adding initial support (available in 18.06) and improving support (available in snapshots) are two different things.
Yes, you can eventually upgrade to a stable build, but this only makes sense once a new stable (19.x) comes along that includes the 3370 commits. Apparantly they came just a bit too late to be considered for 18.06.
That is indeed confusing. According to the changelog, that commit should be in 18.06. Looking at the 18.06 builds, though, it is clear that it is not.
At the moment I'm running snapshot r7054. As far as I can see that's a rather old snapshot from very shortly after the "full 3370 support" commit came along. It runs perfectly fine and without any problems, and I will probably only upgrade once 19.x comes along.
(Also I just noticed that I completely forgot which make my 3370 flash is, so once I want to upgrade I will probably have to open the case again. 
Learn from my mistake and note it on the case or something.)
Thank you for the clarification. 
What disadvantages does it have, if I take a stable build in which the commits are not included?
Is there anything that I can check, that I do not get a very unstable build?
It seems to be daily snapshots, what is the likelihood that I could brick the device. 
I'm using ArchLinux and LineageOS, but I'm still new to OpenWRT and how it works. Please do not mind, if I ask so many questions to make sure.
I'm actually not sure about that, seeing as I never used a release build. Judging from the commits that went into master you would miss out on half of the flash memory space, generally better support, and of course the newer kernel version 4.14 (18.06 is still on 4.9 AFAIK). Generally speaking, right now the release build is about 9 months behind on all improvements that since went into master.
Of course there's always the possibility that something broke in the snapshot you get on a particular day. I have yet to experience that, though.
With this particular device it's practically zero. Since you're using the bootloader to FTP in new images, as long as you don't overwrite the bootloader (and you'd have to do rather silly things to do that) you can repeat that process any time.
Many Thanks.
I still have a general question.
In the following blog post stands:
This means that the luci web interface is not pre-installed and you should only install new packages within the first few days after flashing (so you don’t pick up newer packages incompatible with your firmware).
Does that mean I have to install a web interface later manually? This is quite unfortunate if the device is not configured with the internet ...
https://blog.michael.kuron-germany.de/2018/12/openwrt-on-avm-fritzbox-3370/
Correct, alternatives include waiting for 19.03.0, using imagebuilder with snapshots now and including luci into the image you're going to flash or building from source.
Thanks for the clarification.
When is 19.03 expected to appear?
Do I have to install something besides luci gui which is necessary for normal router operation?
Can I connect the 3370 in the short term behind a 6490 Cable only to install luci or gives that problems due to cascaded router? Because I'm not able to set anything as newbie.
cascaded router shouldn't be a problem for installing packages.