Krube33

i currently have an 07 525i and im looking for advice on if i should buy a new or used m\t to swap for my auto that i have now im hoping to be able to start doing power mods very soon after i do the swap so what ever i do end up going with it would need to be able to keep up with the jumps in power. also should i beef up the drive shaft if\when i do swap it since i'll be dropping a decent chunk of change on the trans and it will already be up or should i wait till i eventually upgrade the rear dif? thank you for taking the time to read this and if i'm trying to do this all out of order i would love a list or guide on the right way to do this i would be very great full.