I've seen retrofits of standard brake ducts to M3 bumpers, in fact, if memory serves me right, Rooz did it. But that's probably different than the M-tecknic kits.
As far as the front suspension, the control arm bushings are sold in pairs, so you might as well do both- it doesn't really make sense to do only one side, knowing at some point you'll have to do the other side anyway. Especially if you're having someone else do it for you, you'll pay more in the long run. Loose steering and sometimes wheel shimmy at high speeds can indicate worn bushings, but visual inspection is your best bet for determining their condition- the rubber will have tears if it's worn, and the control arm will have excessive play when you grab the wheel and move it around.