1. take a look at h&r street and kwv1 and pm vendors here for the best price.
2. on an e90 run e90/92. e91/93 weights are different
3. rear bar is the same. as are all rear suspension components. front is different.
4. a spring/strut combo allows you to pair components that will work very well together. sub-1k coilovers, on the other hand, may be lacking (performance-wise) or better. depends on the setup of course. with coilovers you get adjustability. as long as you get parts that have warranties, i wouldn't be too concerned.