I (well the wife actually) has a Scenic, but a 1.6 petrol. Wouldn't like to tow a RIB with it, and the boot space is shorter than an estate car, but taller.
Daz: I'd go for an estate car. Lots of choice in your price range - Mondeo, Pug 406, Citroen C5. Not much to choose between them tbh, they'll all carry 4 adults and a load of kit in relative speed and comfort. Diesel will give better mileage, slightly cheaper servicing and last longer, but cost lots to fix if they break. A modern 1.9/2.0 turbo will give similar performance to a petrol engine of the same size, and return about 40mpg.
Bigger estate cars can be bought for silly money - Vauxhall Omega is an excellent bit of kit; comfortable, roomy, quick, well appointed, and available in a diesel too, but will cost a bit more to run both for fuel and insurance.
4x4s use more fuel (isn't there a thread about them somewhere
) Boot space is also less than an estate car - Range Rover boot is really small.