when i bought my truck i swapped out all my fluids, had radiator flush, changed all gear/tran/transfer case oils, engine oil, brake fluid. the radiator flush cost me bout 135 bucks. had tons of 80/90 gear oil so i dont really know how much it would cost, spend bout 100 on the oil change, 100 or so to have the brakes done by a professional ( i dont trust me self enough to risk other peoples lives) and the diesel to run it, put over 3400 miles on it since i got, havent had an issue yet, but i saved 1200 bucks on my first insurance payment that an put towards repairs so for me it works out, for you cutting your fuel mileage in half may not be so smart, i only lost like 1-2mpg max.
edit - last fill up took 43 gallons of diesel, i went 404 miles before i had to fill it up. so thats like what 9mpg?