If you are on a budget, for the most part you should stick with stock parts.
1. The simple answer is no, M3 parts do not fit on an non-M. Some parts do, some parts fit in some circumstances, and some never do, so it's a case by case situation.
2. Anything that is "solid" or "track use only" you should avoid. Most stock parts hold up fine for track use and autocross, but many solid track items will not hold up to the different loads in street use. Many of the "solid" items only benefit race/track cars that are being pushed to the limit, and need to prevent their alignment from changing fractions of degrees. Until you are on race tires and are doing custom alignment settings, you should save your money on these.
3. BMW front control arms are frequently replaced when the bushings wear out, as they are press-fit. If your control arms are "bad" you could have a variety of problems related to the steering and front suspension. A search will give you lots of details about this.
4. If your are talking about the sway bar mounts, don't bother unless they are damaged, and if you are talking about the rubber bushings, only replace them if they have damage or excessive wear and slop.
5. Anything with a ball joint should be replaced if it has excessive play, does not articulate smoothly, or shows any visible damage such as tears in dust boots, leaking grease, bends, etc...