Using just lowering springs is a bad idea. It will wear out the OEM shocks because the shocks will have to compress much more to absorb bumps. It's logical; the spring is shorter and therefore the shock has to travel more when you hit [any] bump. That happening all the time means accelerated wear on shocks = you're looking at buying new shocks anyway.
Replace shocks/struts at the same time is cheaper in the long run. You spend less labor getting them installed at once and the shocks last. And you have your OEM shocks sitting around so if anything goes wrong with your aftermarket setup, you can swap 'em back.
The suspension is the hardest part of the car to mod. It is a system - you change one part, it will affect every other part. Like dominos. Legato2057 has the right idea with his kit. That is an economical and safe way to lower the ride height.
Additionally you can look into a low-end coilover system (KW V1 comes to mind). I don't like them because they are unreliable and not good performers, but they are what they are - adjustable lowering kits, nothing more. Personally I would go matched springs/struts, assuming you can calculate out the ride height you want. Can't change it like you can with a coilover kit, as the spring itself determines the ride height.
2011 BMW E92 335i - Le Mans Blue/Black, 6MT, M Sport package