The Alfa should be the 166 TI as it's quite a heavy boat
The Megane discs are 5x114.3 with center bore of 66.1mm, so the same size as the Nissan s14/skyline etc.. hub. Probably the offset will be different than 350z or Evo discs. Knowing what range of ET rims fit to these Meganes I would assume the offset of the brake disks will be bigger than the one of RWD cars.
As for BMC, I would definitely go with bm50 or bm58, although the rear brakes should be upgraded too (to two piston), as the brake bias will be way off.