Welcome to the board and ... good luck with the 540i.
Make sure that the bolts on the oil pump are tight and that the engine mounts and transmission mounts are in good condition.
6 speed transmission are very hard to find and will cost you tons of cash.
Swapping transmission isn't cheap and usually isn't worth the trouble except if it is a rare car.
'85 Euro 635csi Race Turbo, megasquirt, Group A
'92 525i Touring 5 speed, 3.46 diff, UUC 8.5lb flywheel, soon to be turbo.
'02 Subaru WRX Wagon with STI springs, bigger sway bars