1. Are wheel spacers as simple as they sound? Is there anything in particular I should know?
Yes. They are as easy as they sound. I am running 18mm spacers and they look a ton better. In fact, my 70 year old neighbor came out the day after I put them on and wanted to know if I got new wheels because "they look great".
2. What is the largest safe amount to put the wheels out by? 20mm is about the most I can find - will 20mm look significantly different?
Pre LCI is fine with 20's. No rubbing. I wouldn't go much larger.
3. Any types/manufacturers I should avoid?
Make sure you get ones with a hub-centric design. This allows the BMW wheels to sit on the hub as designed. The BMW wheel bolts are not designed to bear all the weight of the vehicle by themselves. They need the hub.
Also MAKE SURE YOU GET EXTENDED WHEEL BOLTS! This is the only real safety concern. If you try to use the stock wheel bolts they wont be long enough. Some manufacturers offer wheel bolts with the spacers, some you have to buy seperately. I used the ones on burgertuning.com and am very happy.
4. Are there any common side effects of fitting wheels spacers?
If you go too large you can rub. I have the sports suspension and have no rubbing with 18's I have seen 20's with no rubbing.