From my own experience with migrating to LR as well as my professional experience on data/file migration, my advice is to import it all into LR first. It gives you a very visual clue of what you have, what belongs together for some reason etc. Just as John says, you can then do something about it, more based on needs than going trough the whole bunch which is probably very time consuming without bringing much value. Also consider a plan how to go about with new images added to LR. These two, your legacy versus your new, can be managed separately in LR.
If you decide, as part of weeding the legacy, that you want to move images to a new folder, please do that from within Lightroom, makes life sooo much easier. An additional nicety when moving images to other folders from within LR, it moves along those xmp sidecars that are of value, and any JPG that is considered to be linked to the raw image. When all images in such a way are moved out of the legacy folder what remains is then up to you to do with it. A much easier task i reckon.