Hmm... if it wasnt a laptop Id recomend getting the Yamaha db50xg... but since its a laptop... nevermind. One possiblity, depending on the speed of your computer, is to get one of these Yamaha S-YXG50 midi softwynths that are XG compatable... these will give you something in the range of 500 voices.... and all you need is a computer with a fast processor and lots of RAM.
Heres a link to the Yamaha page for the S-YXG50. Im not sure how well this will work for composing in Cubase, but its worth a try, and I think there's a free demo to download. Another idea is to get a softsynth such as VAZ or Rubberduck. These are fully midi controllable from Cubase or Cakewalk... for more info on these programs check out Kilo's synth section on this site.... excellent write ups on these programs and lots others...