do I need to put all the above things on?
You won't need the starter relay if there's no electric starter on the CD.
Have you heard of Boyer Bransden Single Phase Power Box
Yes, they've been around for quite a few years now.
If the alternator is a two wire type (AC) then there's no reason why it wouldn't work, but as I have no wiring diagrams most of this is guesswork.
I can get another CB200 engine here :In your opinion,should I rather forget about the CD185 donor bike and just buy this engine? as my current CB200 engine has seized cam....
That may prove to be the better (easiest) option as long as the engine is good?
Worn/seized camshaft bearings were a common problem with these early CB Honda twins, so a replacement engine may not be in much better condition? Otherwise fit the CD185 engine and electrical system as it is, after all, do you really need it to be 12 volt?
I've no idea what parts (if any) can be swapped over between the CB and CD models without actually trying to fit them, as I have never owned or had anything to do with the CD 185 model, and its many years since I touched a CB200, sorry I can't be more help.