If you can describe the vibration a little more and the conditions that exacerbate it, we can possibly help you narrow it down, but here’s a write-up anyways because I was procrastinating work…
Shaking is damned elusive unless you fix it right away. Some guys on BF.C have replaced practically their entire front suspension and keep getting the shakes.
First off, if it becomes significantly worse and pulsates with the wheelspeed when you apply the brakes and it gets worse the more pressure you apply, replace or turn your rotors because they are probably warped. I've had a rotor so bad that it shook during cruising speeds without braking. Never neglect your braking system.
Anyways, It's difficult to isolate the cause of a vibration in a system of many parts where any one part that is bad could cause large vibrations. As you prolly know, generally the thinner the tire sidewall, the more road disturbances you'll be able to notice on the road, I'd try swapping back to your old wheels (unless the tires have been removed or something) and then you can either isolate it to the wheels\tires on your 18" or rule the 18’s out as the cause.
If it goes away and it our 18"s are causing the vibration, go somewhere else with a better balancer. If they have to put a significant amount of weight on the wheel, you will still get vibrations at different speeds because of the tires' apparent shitty construction.
If it's not the tires that are causing the vibrations, first: Get a damned good alignment before you proceed to diagnose the rest of the suspension --save checking visually for worn parts. You should also have the guy doing the alignment also do that for you – if it’s a good alignment shop, they will tell you that you should replace parts if they are worn before you get an alignment. A worn part won’t hold the alignment spec, so an alignment would be useless if something was worn. Misaligned wheels can really cause strange things to happen with the car, especially if your car's toe is out of whack. Hopefully this will fix the vibration. If not, the mechanics didn't align it right or this is not your problem and it’s time to move onto replacing parts!
I would still seriously try to rule out the wheels/tires before you start buying things. (besides replacing rotors) If doing one of these things doesn’t work, move onto the next part to replace.
1. Wheels/tires and rotors. Discussed above.
2. Were the LCA bushings reused (I don't see how you could with the stock ones…)? Really unlikely but you probably have the stock ones on anyways, worn or not. Those stock ones SUCK and I'd toss them anyways (I consider them "worn" right out of the box) and stick solid M3 bushings on. Get the offset ones if you want more caster.
3. Worn tie rods. (replace the whole assembly) These really don't wear out too badly, but they aren’t that expensive to replace and if they are bad, they are bad. Don't cheap out on these though, go for lemfoeder or genuine bmw part replacement. If a cheap tie-rod fails and snaps, you ARE going to crash – and you’ll regret saving that extra $30-50. You can check them by taking a pair of channel locks and clamping on the ball joint and nut that attaches to the hub. If it compresses, it's bad.
4. Worn strut hat's/bearings - get new ones, and you might as well throw in a sport suspension while everything's all apart.
5. Worn steering rack, try replacing the fluid first, it might help, but if the rack is worn or bad, you might want a rebuild or a new one. Quite expensive, if you get this deep into the suspension, there's something really wrong.
6. Wheel bearings - You would probably notice a grinding noise when the wheels are turning, whirring or something strange other than a low frequency vibration - but it's possible.
7. This is in the back of the car, but I suggest you replace the RTA bushings, and get Ground Control shims. MpactMotorsports has a RTA bushing kit with the GC shims for 79$ or something. Quite a deal, instead of buying the bushing and shims separately elsewhere (no I don’t work for them). The shims are very helpful. This would stiffen the rear end quite a lot and would prevent the back end from ‘swimming’ around the road under acceleration, braking, and turning.