There are any number of ways to narrow down results and focus better on scaving for items you want/need. Your suggestion is one. I think it's as good as any and probably easier to code than mine but I strongly disagree with a 24 hour or any re-spawn time. With a re-spawn timer you could very well end up cancelling out any improvements you were trying to make.
Also, allowing someone to increase there odds for nails doesn't necessarily mean that they see only results that allow them nails. My suggestion, also posted previously,would group items into existing player skill pools and allow a player who is, say a master architect, to look for only materials that he would use for that skill (nails, bolts, metal sheets, arch. blue prints, etc.) and the higher the scav skill the better he could narrow his odds to find what he's looking for while still not getting only nails, or one item, as a result. Each milestone skill lvl reached in a skill would allow a second scav button option that could be used to find scav items from that skill group.
As well, to say 'items would need to balanced on rare even more that now' in my opinion would be an error. Scaving now is total randomness. A new selective scav system should be tested first without changing any rarity then balanced from there only IF items came up to frequently within a certain averaged criteria -vs- scav skill level bonus.