It's not really different versions. It is different features.
WebLogic platform 8.1 - it has almost everything included: Server, Workshop, etc. If you want to run something from BEA under Windows you will have to install it anyways.
WebLogic Workshop - it's
IDE for creating applications. It will automatically deploy your project on WebLogic server. I am not sure if you can run things that you do with WL Workshop on different servers
They so much use BEA proprietary things etc
It's included in WL Platform.
WebLogic Server - it's web/app server. Also included in WL Platform. Required by WL Workshop.
In short, probably you will have to install WL Platform. It has everything. Though I think most things are easily done with WL, but you will have hard time using other server than WL.