…for connecting to modern networks and the internet
Farallon AAUI RJ-45 Ethernet transceiver
Because the Quadra 700 and Quadra 900 were made while there were two standard ways of connecting to an ethernet network (Coax with terminators in the ends and RJ-45), Apple chose to think differently and develop a third type of port called AAUI-15 for the built in ethernet. There is an article on Low End Mac about the AAUI Ethernet connector and it's history. Since RJ-45 Ethernet has become the modern standard for wired networking, an ethernet adapter is more or less a necessity if you want to connect your Quadra to the internet or network it with modern Macs or PCs. The other way to network would be to use LocalTalk, wich can be useful for networking with Macs without Ethernet ports (pre-Quadra Macs). There's an article by Adam Rosen on Low End Mac about networking with older Macs.
I found a used Farallon AAUI RJ45 Ethernet Transceiver on ebay that didn't cost much, even with shipping from the USA to Sweden. It works fine and connects my Quadra to the internet connection shared from my MacBook Pro. It's the one you see on the right here. There is also some types wich have two RJ-45 ports that allow you to connect more than one computer to your Quadra.
…for connecting to a modern screen
The other essential hardware addition to a Quadra 700 or Quadra 900 is an adapter from Apple's DB-15 display port to a standard DE-15 VGA port to connect the Quadra to a modern screen. In the past, the manufacturers of screens for Macs included cables for their screens. The trouble is that the Quadra would use how certain pins on the plug are connected in the screen cable to figure out wich screen resolution, colour depth and what refresh frequency to use, so even if you have a cable that connects an older screen to your Mac and the port on the back of the screen is a VGA port, you can't use this cable with another screen with a VGA port if it is not capable of using the exact same resolution, colour depth and refresh frequency. I tried using a cable that came with my Radius 2 page greyscale display (21 inch) to connect my Samsung SyncMaster P2770HD screen/TV to the Quadra and the result was that I could only use 16 greys in 1152 x 870 and the screen looked very blueish. Even if it is possible to get a Mac DB-15 male to VGA DE-15 female adapter "block" such an adapter would not be of much use since there is no way of knowing wich resolution and colour depth it is made for when you buy it, except if it is stated by the seller. It might not match your screen at all and you will have no flexibility if you need to connect to other screens with other resolutions in the future.
After some research, I found that the Viewsonic Universal Macintosh Adapter VMAC-1 that has DIP swithces for changing resolution and sync mode would probably be one of the best options. I found this user guide online. I got one on ebay that was unused and in its original packaging. It has DIP switches for setting the resolution and sync mode, but these can be hidden by a lid when set. With this adapter I have been succesfully using 16 colours with a resolution of 1152 x 870 and a 75 Hz refresh frequency on my Samsung screen. The screen seems happy with the sync mode set to either Separate, Composite or Separate+Composite. Lately, I have switched to 832 x 624 to get 256 colours. (I have the stock 512 kB of Video RAM, so I can't get more than 16 colours at 1152 x 870.) 16 greys was fine on the Radius screen, but many programs seem to anticipate either black and white or 256 or more colours. My Samsung SyncMaster handbook does list Mac 640 x 480, Mac 832 x 624 and Mac 1152 x 870 in a "Table of standard signal modes" together with horisontal and vertical refresh frequencies and polarity, so maybe that is why it has worked so well for me. It doesn't list the other resolutions the Quadra 700 support and I haven't tried them either. "If it ain't broke…"
For more information about the internal graphics card in the Quadra 700 and Quadra 900, the article by Dale Adams, the original designer of the Quadra 700 and 900 built in video capabilities is the best place to start. There is also an article by Glenn Fleishman on TidBits discussing the advantage of the on board video of the Quadra 700 over later computers in the Apple lineup. Where to find these things in Europe
Places to look for items like these in Europe would be amazon.de, amazon.co.uk (they do not ship electronics to the rest of Europe for fear of mismatched power outlets, so if you don't live in Britain, use amazon.de in stead), finn.no, blocket.se, dba.dk, ebay.se, ebay.co.uk, ebay.de &c There are lots of cheap items from the USA on ebay. These can usually be shipped quite cheaply if they don't weigh much. If they are not too expensive, you don't have to pay customs either. For more costly or heavier items, it is usually better to shop in Europe since there is no customs inside the EU and shipping is less expensive. It's also better for the environment not to ship lots of heavvy goods around the world.