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Nothing is lifetime - not even your transmission fluid.
B6 & B8 dampers had some big issues on E8x/E9x BMW's; there are countless threads over at German BMW forums about it, yet Americans say their Bilsteins last long. Some guys had to get new shocks after 10k miles, some have had no issues whatsoever. However, it seems that standalone shocks like the B6 or B8 are more robust than the ones in the B14 coilover kit.
Just a heads-up.
As for your question, Bilstein says B8's are for M Sport springs. You could pair both HD or B8's with M Sport springs without a problem. The B8 shocks are 30mm shorter, I believe.
The B12 kit is a great bang for the buck; comes with B8 shocks and Eibach Pro-Kit springs. Search around, lots of info here.
P.S. 70k miles on Konis is great; shocks on an E91 were gone in less than 60,000 miles.
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