How much time do you recommend staying in Berlin?
Would it be safe to camp on a roadside, even if it is in a non-touristy area?
Finally, would £2,000 each (€2,400) be enough to cover everything in the trip inc. fuel?
Five days would be reasonable for Berlin. Depends upon how you manage with the car and money, because for Berlin tent and car is a bad combination. You'll most likely end up in a hostel and pay for public transport as well as parking the car.
It is actually easier to camp roadside in a non-touristy area. In a touristy area the local authorities will drop by and tell you to go to the next camping site (especially in France, Netherlands and Italy close to France) and in some cases fine you for camping wild (especially Netherlands). In non-touristy areas they couldn't care less as long as you are gone the next morning. (exception: the Netherlands, here staying on official camping sites is mandatory)
2000 pounds each for 4 people (8000 pounds total) with cost of fuel shared between them sounds like a very good budget for 3 weeks, especially if you camp wild occasionally, buy food from the supermarket and cook dinner yourself on a small camping gas stove or at the hostel kitchen. Chances are high you'll come back with some money left in the bank. Around 1500 pounds per person will work if you are a bit careful with your spending, 1200 pounds per person should be the ultimate minimum for 3 weeks.
[ Edit: Edited on 10-Jan-2011, at 17:45 by t_maia ]