Hey there,
im looking for a router to install openwrt to use it for ad blocking and ftp server. to save files over ftp the router should have usb to store uploads from a ipcam. it would be nice when the installation process is as simple as possible, no tftp or else. it should have enough flash and ram for my purpose.
price range is up to 100 euros.
any suggestions? thanks a lot 
This MR8300 seems a good choice to you.
Recommendation would depend on the speed of your internet connection, wouldn't it?
Would also help to know if we're talking about a router or a combination router and wifi access point (i.e. what non-nerds mean by "router") It changes the answer a lot.
The device is just for ad blocking as dns server and ftp server for private use as only one user, no heavy traffic. AP isnt necessary, i already got 2 AP at home with mesh. my connection is 100 mbps
At $35 USD, a Raspberry Pi -- even a 3B+ which you can currently find in stock -- will outperform most inexpensive embedded routers, and it has a console and removable storage. If you want durable local storage for FTP service, an SSD with USB adapter will do it. (A Pi 4 is better in that case, given that it has USB3, but even USB2 storage will more than saturate your connection.)
You're definitely better off with RPi4 4 or 8 GB RAM and use a TP-Link UE300 USB3 ethernet, works like a charm, reliable and does not overheat.
Adblocking can be quite RAM hungry depending on your block lists, even 512Mb RAM may not be enough, I run over 1GB RAM used on my RPi4 w/Adblock (don't forget Adblock blocks domains, so it can also be used to prevent inadvertent access to malicious/malware sites) and I use my MR8300's as AP's only, descent AP's though.
Hmm...? I'm looking at my blocky instance (which is go so not extremely memory efficient) and it's using 41Mbyte of resistent memory so I would imagine that 512Mb would be plenty enough.
Depends what you block, if you block basically nothing, this the memory footprint is nothing...
Right... even the full oisd lists are 24Mbyte so I'm not sure what you're using
I agree on not skimping on RAM too much, but 1 GB RAM is plenty - although I'd still pick 2 GB for good measure.
total used free shared buff/cache available
Mem: 3929316 55248 3318984 3344 555084 3795900
Swap: 0 0 0
(x86_64, luci-app-adblock with a quite reasonable selection of block lists, no wireless)
admittedly, there are corner cases when dnsmasq can eat RAM for breakfast (tcp requests).