As far as the swap goes, a 6-cylinder with a 5-Speed only weighs about 50 lbs. more that the stock engine with an automatic. The auto tranny is just plain heavier.
Never seen a turbo kit for the M42/M44. But then again, I've never looked
As far a "kick" goes, you will not be happy with an auto, so thankfully you're getting a 5 speed. I've driven both and there's definitely a difference.