1) It is good to stay close to the original offset, although in my experience it does not have to be exact. Aftermarket wheels usually run lower offsets for a more aggressive look (pushed out a little more). Within reason, this does not cause problems.
What actually fits is important as well. A wheel has to clear the calipers, not hit the fenders, and not hit the suspension. For a given width, this restricts the offset that can be used to within a certain range.
2) Spacers allow wheels to fit on a car that may not otherwise work, or they can help fine tune the look of the car, putting the wheel exactly where the customer wants it. The downsides are insufficient thread engagement (unless longer lug bolts are used), and more strain on the lug bolts. Cars that are run on racetracks, should change their lugs regularly (once a year or so) to avoid metal fatigue resulting in a broken lug. Like a shark attack, this is very rare, but has been known to happen. And like a shark attack, the results aren't pretty.
If you can fill your needs without a spacer, do so. If you must have a spacer, be smart: get proper length lugs for proper engagement, and replace them regularly if you run on the track.
I have accepted a new position at Tire Rack and am no longer in the sales department. firstname.lastname@example.org
is available to assist with M3 questions.