I love the 6ers, my friend is building a turbocharged 3.5l M6.
I will probably pick up a clean 635csi down the road... but it will have to be 1985 and later.
The older ones are not based on the E28 like the "newer ones". They are based on the older E23 if I am not mistaking. with some weird suspension setup. Doesn't handle as well as the newer E28 based 6ers.
'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