Hey Gnair8,
I think the E60 can easily accommodate the 20 incher.It quiet actually depends on the the kinda suspensions you're on.If standard Highline or Msport,I think there's no issues but if you've gone for some serious lowering than you may have to consider rolling your fenders,something that most high end mod shops can do easily these days coz they now have a special tool that hooks up to your wheel hub without having to do any fender banging or touch ups.
In my case,the car came with 19" Hamman PG3 2 piece which had a crazy offset and initially The tyres were rubbing my fenders and I got that rolled and it was still touching,I was at my wits end and the next thing you know,It stopped doing so.Apparently my suspension was sitting loaded for a long time and it kinda sprung back to it's original ride height after a few thousand km.How that happened?beats me but I'm glad.By the way,I have the Mtech suspensions.In my opinion.the 20s look great but you may want to look at other aspects before going for them namely our road conditions,ride comfort,tyre prices and availabilty.I've got a 530 e60 too and i think 19'' is just fine in terms of looks and performance.Had it been a 545 or the 550,20" is almost a must