I have been looking for an integrated scoring editor within Cubase 7. Its quite a dream come true that Steinberg actually provides one now, Dorico.
Since I already have the Instruments with me, can I have just the Scoring Software sans Instruments ?
Cubase of course already has an integrated scoring editor, which it has had for more than 20 years. If you need help finding it, please do ask in the Cubase forum. It’s very capable in its own right.
When you install Dorico, it won’t install any of the HALion instruments and content if it finds them already installed on your computer.
May be I’ve not used the full features of Cubase Scoring Editor. Can I work with keyboard or something with inbuilt score editor ?
Yes absolutely. You can even play freely, then edit the result to get a decent score. I prefer Step Input, obviously… 
As the composition progresses, writing score along the way will be very important. I don’t think internal score editor lets me do that like MuseScore.
I don’t mind buying Dorico given all that its letting me do. But I guess the pricetag includes the instruments I already have 