Hey Mate
I actually wrote you a fairly lengthy response yesterday, however I got distracted with um er work and clicked off it before I submitted it...
As far as building it is concerned, it depends what you want. Some like a full high end build with all Cook Bros Components, like mastertints insane examples, they are totally stunning and a couple of the nicest builds I've ever seen...
However, when I set about aquiring a CBR I came across a very nice example and bought it complete. When it arrived it had higher spec components than I had on mine. However, because I wanted to replica my original bike (from the attached polaroid), I actually removed those components and built using the original components, I'm so glad I did, because it means so much more in the memories when I look at it in the format I had mine back in the day.

How it looks today....

As far as putting it together, the main thing I would suggest is to take care of (as with any build)is getting the bottom bracket and headset cups pushed in gently and using a little grease to help them slide into place.
You could use the parts from your Mongoose, but I would tend to look for nice condition parts to build your frame and forks as they are high end bike and deserve reasonable quality components if your budget can allow it.
I look forward to seeing some progress photo's.
Cheers
Stu