ADAM,

7,5x17" are "small" enough to make some lowering possible on a W124. H&R should be good, especially if you are going to add sport shocks.
I don't know what you want to achieve with the lowering...if you only want to have a nicer look and are a decent driver changing the springs should be sufficient.
However, if you are a little less decent driver and want to create a possibility to throw your car through any possible corner (not to be considered common to a Benz driver) you will need the sport shocks as well.
The spring pads you want to use depend from the current installed ones and the level of lowering you have in mind.
The dealer can tell you by chassis# which pads are installed on your car. The H&R specs will tell you how much the car is going to be lowered and any change of spring pad will bring you some additional lowering.
If you don't want to go through the dealer/pads trouble, just use the #1 pads and you will get the car as low as possible with the purchased H&R springs.
If the lowering is to much, you can always install the original pads again...

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)