i generally just pressure wash round the outside of the engine bay, look on ebay for a autosmart G101 ( i would sell some but only got a little left and cant be arsed to go and buy some of my supplier lol) use this as it is a really good all purpose cleaner and good on grease and oil, spray neat or 50-50 with water and use a little brush to stimulate the cleaner, then rinse with jet wash or just use a cloth afterwards, then go over all your plastic with a dressing and it will look mint then and use autosol on the aircon pipes, they come up like chrome then
here is mine after about an hour spent on it:
