I'll try to answer your questions in order.
1) Every after market hood will require modification or work to install properly, wether drilling holes or trimming sides or such. Simply because most aftermarket hoods are "hand made" so not every single one is turns out to be the same.
2) I highly doubt any BMW dealership would install an aftermarket hood for you, your best bet would either be a very reputable automotive collision shop or a reputable BMW Indy shop that have installed aftermarket hoods before.
3) As far as paint goes, we do offer pre-painted but it really depends on your car's paint condition currently. If it's faded and hasn't been well maintained you may need to have the hood painted locally to have it blended in with the rest of the car, but if you've taken care of the paint and don't leave it out in the elements too often then you could go with a pre-painted option.
Hope this helps, feel free to PM or email me if you have any other questions.