I just had to do this on my 2005 X5 and I replaced the rear lines with AGS NiCopp. Supposedly this copper alloy has been used for brake lines by other manufacturers for decades. (Audi, Porsche, etc.)

I used 2 - 25 foot rolls of NiCopp and ran each line as a continuous run. This should cost around $60 online (~$30 per roll). NiCopp Nickel/Copper Brake Line Tubing Coil, 3/16" x 25' - AGS Company AGS Company You'll have some tubing left over from each roll but a single 25 foot roll is not enough to do both lines.

I ended up using an HFT50 flaring tool after buying a cheap one and being disappointed with the quality of the flares. The HFT50 is not cheap ($140) but it is a quality tool. http://surrauto.com/documents/HFT50H...laringTool.pdf Practice! Practice! Practice! before making the flares on the car. Also note that the flare type you want is a DIN Mushroom flare. A bubble flare may work but it's not the correct flare type.

Most of the bending I did by hand but in the wheel wells where the connection is made to the rubber brake line I used this bending tool from Harbor Freight for $10. 1/8" to 1/4" Tube Bender
It gave nice consistent bends and is a good size for fitting in tight areas if needed. I also used it for making the bends going into the distribution box.

Once I got the NiCopp lines in place I bought 4 feet of 3/16 ID rubber hose and cut it in 2" or 4" sections and slit it along the length and used it as additional vibration protection bushings for spots where the lines may cross or they may come in contact with the body. These were tie wrapped in place.

FYI BMW sells the steel lines cut to length BUT that will cost you about $200 for all the lines. But the big problem is going to be joining the lines that run under the trunk over to the right rear. Getting a wrench in there will be too tight unless you connect them before installation. Using a continuous line avoided that headache as I just fed the line from the left side over to the right side.

Also be aware that BMW recommends using Low Viscosity DOT 4 brake fluid. I chose to use BMW fluid from ECS Tuning in 12oz bottles. It's cheaper to buy the 12oz bottles than a gallon and if you dont use them all they will stay sealed and be good for your next brake flush.

WARNING! My lines rusted out in the usual spot right behind the drivers side wheel well under the plastic cover. However I noticed significant corrosion on both lines hidden behind the left front wheel well splash guard and on the right rear line as it travels from left to right under the trunk and over the differential. Just checking under the long plastic cover is not enough!