The running & maintanence costs are about the same as a 2wd Golf of the same age/mileage, the fuel consumption is a couple of percent more with the 4wd though.
If you have a carburettor car? fitting a Weber carb increases both power & economy
As for value, a low mileage one in tip top condition with full mot etc will fetch round about a thousand pounds or so, but UK prices for good cars do appear to be on the increase.
Have a look through the site you will find the answers to most of your questions.