Please, I need your tip before lowering my BMW E61!

[EastSide]

Hi,
I have a BMW 520D Touring 2006. I have bought new 18" rims and want a lower profile on my car. This is the first time ever I'm trying something like this so I need your advice.

I've read some on the web and found a lowering set from H&R that sound good from what I can tell. It is their sport springs with part number 29255-3 that lowers the car 35 mm.
This set only contains front lowering and from what I've read the rear lowering is made through the internal system.

Is this set the only thing I need? Will the standard suspension still work and is the H&R set a good one? Will this make the car very stiff and if so, do you have another set with less mm lowering (like 20-25 mm) to recommend? Is there something else I need to consider or that I should do?

I'm sorry for a simple question like this, but I've never done any type of modification like this before.

Looking forward to some good recomendations from you experts! I live in Sweden and would appreciate any recomendations on a national reseller that carry suitable parts.
Thanks!