for free orchestral sounds I recommend getting some soundfonts - check out hammersound and blackattackbitch's forum thread: http://www.newgrounds.com/bbs/topic/1200140
(for percussion I suggest Roland orchestral rhythm, that was the only one I had a hard time finding :P)
if you have the cash and are serious then there are lots of VSTs out there for various stuff you want (here's a few: session strings; east west ql silver/gold/platinum <-- silver being the cheapest; LASS; hollywood strings; stormdrum 1/2 <-- percussion) they're the ones off the top of my head right now
to play soundfonts I think FL can do it already but you'll need them as an .sf2 file - if FL can't do it then get the free kontakt player and use that :)
hope this helped :)