I've done the trip 5 times now. With 2 drivers, just stopping for food and fuel, you can do it in about 3 and a half days.
If you have a look at Google maps, or ViaMichelin, you can see a couple of very goods routes which are roughly a straight line as far as Istanbul. Between Calais and Istanbul it's 'A'class, dual carriageways or motorways virtually all the way. Then, once you reach Edirne, you've got a choice of the 'mountain route' or the coast road.
You'll need to buy a Vignette at the Austrian border which covers your road tax. You may also need to buy one at the borders of Serbia and Croatia if you use those routes, but I can't recall.
You don't need any Visas but you need to make sure you have at least 6 months to run on your passport. You'll need the original V5(C) for your vehicle and it must be in the name of 1 of the car occupants. You'll also need Green Card coverage on your insurance which must include Turkey. If you can only get European coverage, then you'll have to buy extra insurance at the border when entering Turkey. You'll need to cross the border during office hours to make sure the insurance office is open because, depending on which entry point you use, the opening times can be a bit hit 'n' miss.
You've got to weigh-up the cost of fuel. You're looking at roughly 5000 miles for a round trip. Say at 35mpg, that's 140 gallons.
There's a huge amount of information about driving in various European countries available on t'internet.
