the trade of with non oem rims is that generaly replicas are heavier and weaker than the genuine article.Plenty of people here run non standrd rims.If you live in an area with bad roads you will almost certainly bend a rim or two,thus setting yourself up for a front end shimmyYou are pushing the envelope for an e34 by choosing an 18 inch rim
You will need a hubcentric ring if the wheels are suitable for an e39.They look a nice wheel tho