2005 177K miles service repair questions

Hi Everyone,

I have a 2005 (with 177K currently) that I will be honest, since we bought the car used in 2007 with 38K miles I have only had oil changes, TSB's, engine and cabin filters, and front brake pads replaced. During the most recent visit to the dealer for an oil change they said that the car needed its inverter fluid changed and rear brakes replaced because they are at 15% wear remaining. They also said that it should have the spark plugs and wires replaced ASAP. In recent months I have noticed a decrease in mileage of approx 3-4 mpg over the same months in previous years (we live in Indiana and the winter is COLD and nasty). Could someone please help with what service(s) need the most attention and help with and what my handy husband may be able to do himself? He thinks the rear brakes won't be a problem, but I know that the Prius is a little more complicated than other cars. Any help and advice is MUCH appreciated! I need this car to last another 50K and then I get a V Thanks!