As far as i have been told they do not affect breast feeding at all as they do not touch ur milk glands.
Overs is when the implant is placed over ur breast muscle (like ur breast tissues sits now) and unders is when they go under ur breast muscle, this usually happens when you dont hav eenough breast tissue for the size u want :p.
Both have pro’s and cons which i am sure u will be told at ur consultation in detail. As with everything everyone has their own opionions and some people think over r more natural or unders or more natural but to be honest i think it really just depends on what size u get and what figure u have at the moment.
At the end of the day u say what u want and ur surgeon will help u achieve it 
Hope this helps lol xx