1. If i was constructing a model to 1:200 scale i would use 3mm thick MDF board throughout. That would be easy to cut using a Stanley knife and when'boxing in' between the ribs this would increase the strength of the hull.
2. I have always used Robert Hahns' plans. I think you can still buy them online but i dont think you can download them.
3. I use quick drying ready mixed Polyfilla. Easy to sand and provides a smooth surface.
4. As mentioned i use 3mm MDF for bridging the gaps. I even used 3mm between the frames for my current 1:72, however i used 9mm MDF for the frames and keel plate. 9mm would be to thick for your scale.