Would there be any issues if I create some 600 keywords in LR?

Question

Im trying to find a way to store the original final name (from before import/copy of pictures in old pre-Lightroom folder structure to new folder structure).   There doesnt seem to be a metadata field for 'source folder', so it seems the best I can do is manually set a keyword along the lines of   OrigPath=E...\Nikon\folder
The catch is my current photo library is spread over about 500+ folders.
One issue I can see is that they will take up a lot of space in the keyword list which might make it harder to use the dropdown etc for setting other keywords
Im using keywords as they can be directly editied in Windows Explorer Properties etc (as 'tags').
Is there a better way?

Answer 1

Ok, step by step.
The current folder structure is haphazard.  Likewise file names which include duplicates (for many reasons, multiple cameras with similar numbering, folder partial duplicates for making albums for distribution etc).  Some 12,000 files accumulated over 10 years or so.  
And now I'm starting with Lightroom.  I think I've figured out a workflow going forward ... Thank you DAM book!
So, first assumption... It's a good idea to import/copy the old library into the new folder structure.   Is that a bad idea?  As opposed to basically running two libraries which will complicate archival and backup.
The file names are changing to ensure uniqueness in the new folder structure.
old1\a.jpg.    and.  old2\a.jpg. may both be copied to new17\....    or whatever.
(And also, consistency with the new file name convention).
Second, a point of principle.  I hate discarding information.  Some of the old folder names signify 'collections', though most dont.  So filtering on some of the old folder names will help categorization in lightroom.    As opposed to starting over.
Third, can I be sure I will never need to locate the original file?  
Windows Exploder... fair question.  Basically I'm still easing into LR and need training wheels.  Albeit limited ones as Exploder doesn't read RAW metadata.  (But 80% of the old files are JPG ).
So....
First, what is the appropriate metadata field for this?   Exif.ImageID sounds plausible.  Is that right?
Second, is there a way to automatically populate that field on import?
At this point, this is where I'm at... (Lacking answers to these questions and based on the little I know so far)....
Import/Add the old library into LR using a temp catalog, setting a keyword (PreLR)
In LR, folder by folder, (yuk) set a keyword "under" PreLR with the folder name.
This is where the 500+ keywords comes in.
Using hierarchical keyword is a way to hide them when needed (most of the time, going forward, I hope!).
Write the metadata back to files/sidecars.
Reopen main catalog.
Import/copy to new folder/ filename structure.
Archive old library offline and delete.
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