Sorry for your bad experience. However, having both a B4 and a '96 Ducati 900SP, I can say for certain that the Ducati is no less maitanance and the parts cost a lot more. I got my 900 SP used with 3000 miles on the clock. I have had to replace the frame due to cracks (Ducati provided the frame, I provided the work), I have had the engine completely disassembled to replace transmission parts that siezed. I cleaned the tank, replaced the gas cap assembly US$200 and the normal stuff like tires, carb overhaul, battery, etc. all within the last 2 years. I am about US$1000 into repairs with me doing the work.
Most everything you described with your B4 is likely a result of the previous owner(s) not performing proper maitanance and storing it outdoors. Like with my Ducati, the problmes you have are typical of the "risk" you take in buying a used bike over a new one.
I truly hope you are able to get it all fixed up and are happy with it in the future.
Russ