18mm will work fine with a Glidecam. The less wide your lens is, the more difficult it will be to attain smooth shots. However, if you practice enough, using longer focal lengths on a Glidecam an be very rewarding. I.E. your lens will work fine.
I use a Tokina 11-16 on mine, and I shoot at 16mm almost all the time. I've also used a 28mm with beautiful results. I've even seen people get great results using 70-200s (granted, they had a HD4000 and v-10, but still).
As for the weight issue, you can can technically use either the 1000 or 2000. With the 1000, your camera will be an optimal weight, but if you ever want to add a heavier lens or a mic, it will suddenly be too heavy. And believe me, you will eventually get a heavier lens.
With the 2000, it will be a bit too light without a heavier lens or some accessories. You can still mount weights on the top and it will work, but this is generally seen as a less-than-optimal workaround.
Basically, with a 2000 you will be future-proofing your kit. With a 1000, you will be restricted to the setup you have. If you don't plan on getting a nicer (heavier) lens or adding any accessories, get the 1000. If you do, get the 2000.