1. You will not deal with your dealer in most cases to buy out the lease unless they are offering a better deal that BMWFS. Your dealership becomes a middle man and we already know how the middle man stuff works, they want something out of it. So unless your dealership can offer you better numbers than BMWFS, you will be talking with Financial Services.
2. Small mods on leases are ok, large mods will cost you more in the long run considering the amount of money it will take to install and uninstall the mods. If you will mod a lease, do it early in the lease so that you can re-coupe the benefits longer.
I'm not here to give you advice on leasing vs. purchasing but I did try to answer your questions