Would like to request for options to narrow down the target of searches to either selected book/s or OT or NT only.  Would this be possible?

Yes.  This should appear in an upcoming version.

 Are we going to be able to keep one version as a default search book and not necessarily the one we're reading at the time?  I always read the NET bible, but always want to search the KJV, since I usually remember about what it says in the KJV when I want to find a verse pretty quick.

In the current release (and the 4.0 that has been submitted to Apple) searching is limited to the currently-selected volume (NET, KJV, etc.)

We're considering a future upgrade that would let you select a range of volumes to search.  This is pretty complex, though, so it's still in the design phase.