That could be many different things, 90% are expensive...at best it might be a throttle positioning sensor which could explain your high idle, but it could be major engine failure....if you are flooring it and it isn't going anywhere then it is more likely your transmission is crapping out. The price seems reasonable I guess, but you haven't mentioned mileage... buying a used Disco with over 120K miles and no service record is a gamble at best. My advice is to take it to a knowledgeable shop (not necessarily a rover dealer) and pay for an inspection and a carfax. If it ends up being a mess you will thank yourself later....