Consider that if you have a Hartge suspension the springs are likely to be Hartge as well. If that's the case they are set up to compliment each other. The Boge units, whilst a very good product, may not have the same valving. If you are looking for a cheap solution without worrying about the consequences, by all means buy the Boge units. Good quality and should be a good match. Another consideration is how much has the Hartge spring set up lowered the car. Are the shocks matching the lower set up. If not they won't match, Be sure to address all these questions or you will have a problem.
Hope this helps,
Geoff
Sydney, Australia
'94 525iA