Been there,done that......
5x120 is for BMW.I did have Breyton wheels on my C5 but they were also for BMW.Had 9J in front with offset of 38 mm and 10J in the rear with 42 mm.
Had to take smaller tires because of the wheels sticking out of the fender.
(235 front 255 rear)Only lugcentric wich gave a huge inbalance when driving.....

If you get some wheels make sure that they are lug- and hubcentric.
BMW has the closest wheel pattern to the C5 but I sold the Breytons and put on some OEM wheels....WHAT A DIFFERENCE.
Greetz Erik.