Re: some random ideas, comments appreciated
Sorry to hear of the problems with your Z4. I traded my '03 Z4 3.0 in on a 128i convertible with MT and ZSP last April and I'm very pleased with it.
Not caring for turbos, I did not test drive a 135i but I'm satisfied with the 128i performance - it's about .5 second slower to 60 mph and doesn't have the low speed torque of the Z4 3.0, but the gas mileage is equal to the Z4, even though the 1 is about 400 lbs. heavier. I find the handling to be very good (stock SP tires are V rated Goodyear NCT5 RFT). While one is aware of the added bulk, the 1er seems to me to corner nearly as well with a planted feel. The only downside is some additional understeer, exacerbated by the staggered wheel/tire sizes. And the ride is much, much smoother than the Z4.
As my wife and I used the Z4 mostly for touring, the additional trunk and interior space of the 128i is appreciated, but the 10" longer body isn't much of a problem, although I would recommend ordering the PDC (rear camera) as rear vision in any droptop is compromised. The sport seats in the 128i are more comfortable for long distances than the standard seats I had in the Z4, and getting into and out of the 1 with the top up is so much easier on my arthritic neck!
As I only put 44,000 miles on the Z4 in 5 years, I can't comment on reliability (it had the normal warranty recalls but only one problem out of warranty - a defective gas cap). So far, with 13,000 miles in about 10 months, the 128i is the first new car I've bought (going back to 1965) which has not had to go back to the dealer for any warranty repairs, or even any adjustments. I'm quite impressed with the build quality. Note that two problems have been reported by some owners, a cracking third brake light lens (sound familiar?) and (more serious) a failing high pressure fuel pump on the 135i models only.
Hope this is of some help.