Ah ha! Right in my territory!
The standard 8-series wheel was 16" with the option of 17" BBS split rims which look good but corrode quickly.
Any wheel from an E34 (1988-95) or E39 (1996-2002) 5-series will fit tho' the E39's will require adaptor rings for the centres. The 7-series is also a source.
17" look good, don't spoil the ride & handling & tyres are a reasonable price. 18" are more of a compromise & tyres cost a lot more.
The 8-series is a good source of wheels for 6-series owners.
My 635 has 235/40 on 8x17 & 255/40 on 9x17 from an E39 Sport. Full set of G/Year F1's cost under Ԛ£450, wheels were Ԛ£550 2/hand refurbed.
