I think my Head Gasket has blown. Milky grey sludge under the oil filler cap and coolant has completely disappeared, dipstick looked very odd and running very hot. I'm not remotely confident in my ability to resolve this as a keen to learn but low skilled DIY'er. How much money (as a ballpark), am I likely to need to borrow off the wife (reduce the cost her birthday present by), for a garage to do the work?