Orchestral sounds is something you'd best look elsewhere for. Depending on your needs, something like the Kontakt factory library (part of Native Instruments' Komplete; I highly recommend you get Komplete rather than just Kontakt) or an old library like Miroslav might be good enough. I use them - but I don't make much orchestral music. Or you might want to save up a few thousand bucks for the really big orchestral packages. It's difficult to say without knowing your needs. And none of that's free.
Logic should have everything you need when it comes to electronic sounds, though.
If you're only a few years into this stuff, just now going from GB to Logic, I'd recommend something like this:
1) Learn Logic's instruments, how to get a decent performance out of them. This is free, now that you have it. Find their limitations, and figure out what it is you need from future purchases.
2) Get Native Instruments' Komplete. You get Kontakt, which you'll probably want for future libraries, as well as its basic sound library which includes a wide range of instruments and a lot more options than Logic's own sampler has. Find its limitations, and figure out what it is you need form future purchases.
3) Decide whether to build a library out of smaller purchases (that usually run in Kontakt) or one big monster of a library (often its own plugin). If you're not at this point drawn towards orchestral music, you might find that you need better drum samples, or a good fake/sampled guitar, or more synths, or brass for jazz, or... something else. Figure out what you need before you start spending any money.
While there is free stuff out there, I'm not sure I'd bother with it myself. These days, the tools I use the most outside of Logic's own stuff are Pianoteq (mostly the e-piano add-on), Omnisphere 1, a couple of things from Komplete, and a couple of SampleTank things. That's for e-piano, pads, various uses, and drums, respectively (with overlap). But I started with just Logic's own stuff, and that was years ago, before Sculpture and Alchemy. I think my 5 first remixes were done with just Logic's built-in stuff back then, though I can't say for sure.
Sorry to not answer the question, but it's difficult without knowing your needs. Even then, it's difficult to say whether a new tool is the right way to go, or just more skill with the tools you've got. What do you want to make?