1) with Designer/ PowerDesigner, can I independently model my schema (on the normal desktop) , take the script and run it on the database,
2) PowerDesigner, Can I download it from the net.
with Power Designer you can independently model your schema (I am not sure abt Oracle Designer). Power Designer can be downloaded from http://www.sysbase.com and is available for evaluation (I guess 45 days, evaluation period)
The major contenders out there right now seem to be ErWin, Visio, Embarcadero, and Power Designer. However, they all fall under the same general category. ie Not Free. If you are simply looking for an eval copy, you can get that from any of the companies, including ErWin. I'm not sure why you excluded ErWin, but if it was because it's not free and you don't like eval copies, then none of these will work for you. Mind you, if your company does anything with databases, then it *really* needs to purchase one of the above tools. A database without a maintainable copy of the data model is simply asking for trouble.