In hundreds of thousands of miles of driving, I have never had a car that needed front and rear brakes at the same time. All needed fronts much sooner. This cost probably can be spread out over a couple of years, depending on how much you drive.
I just bought a set of front pads for my E90M3, not because it needed them, but because I wanted a slightly higher performance pad (Stop Tech Street Pad). Cost was $85 shipped. Fluid is $12 a liter for something like Ate, but you can spend $40 or $80 if you want. Sensors are not big bucks. Braided lines if you want them are $125 a set (I bought ECS). I have not looked up the price of front rotors yet, but those are the big ticket items. Get the part number and shop around. As people say, find someone to help you DIY. Brakes are pretty easy (except for those two inner braided hoses in the rear . . . .).