Of course, you can be as severe as you want to be in your pruning, but to state the obvious the more you decide to keep the more space you will need.
Typically for a keeper I would store the raw file (plus any sidecar file generated by the raw software which contains the edits I performed), together with a high quality JPEG (and, if I submit a picture to BLP, a smaller file that fits the BLP size requirements). People will differ in this depending on how they prefer to work; for example, some people choose to work with Adobe DNG files or TIFF files, and perhaps keep those with (or instead of) the original raw. This could add significantly to storage requirements particularly as TIFF files can be several times as large as raw files.
Certainly one option would be to move all your raw files off the computer. The size requirement for storing raw is much larger than for storing JPEG (perhaps 3x). In many cases, you wouldn't go back to the raw file often, if at all. But in my view it's important to keep it in case you need to process the image again at some stage.
One thing to check (and I don't know the answer to this) is the impact moving files around has on your Lightroom Catalog (adopting the US spelling). It's ages since I used Lightroom but my recollection is that it can be a real issue if you move files around after processing in Lightroom. This had something to do with the way Lightroom stored information in the catalog about the file - and I would find that the catalog then complains about missing images. It was possible to realign the catalog, but I recall it being a pain to do so. Someone who is expert in Lightroom can hopefully explain how this works (and it may turn out to not be a real issue). One of the reasons I stopped using LR was because I got tired of being locked in to LR by the catalog. My current software relies entirely on the native Window's file system. This has its limitations, but I prefer it for several reasons, particularly greater tranferability between software programs, and more direct control and organisation by me.
Simon