I haven't used iPhoto (or Photos) but I think they both use the same location to save their images/photos. iPhoto just didn't like to show you! At any rate, you probably have at least one 'file' (it's actually a directory) in your default "Pictures" directory. Ot you can search for an item with a suffix of ".photoslibrary". If you simply open Photos, it will 'grab' your iPhoto Library, update a copy and create that new directory for you. Inside that new directory (Show Contents), you should eventually find one called "Masters". You can then copy or duplicate them into any place you'd like and then tell LR where that is.
LR couldn't care less about where you store stuff, as long as you don't change titles it outside of LR. Always use LR to change names of files or suffer the wrath of Adobe the next time you try to open it in LR!
Other than finding your images, I'm not sure what kinds of problems you are anticipating with "using LR". However, it is not iPhotos/Photos on steroids! It will require some rethinking and careful thinking about how you organize your collection. The work flow is 99.8% different, also. It is actually designed for professionals or serious photographers. Be advised that while Adobe still sells a desktop version, it will probably not be updated as often or as completely as the subscription (CC) version. I'm happy with sticking with version 6.0 on my machine.