What he said. Everything that came with KF is fair game, but everyone's using it, obviously, so it may look over-used now. Your choice though and if you think you can do something original with it, go ahead.
Custom maps usually come with a readme in which authors can specify if they want to allow you to use content from their maps. If they don't have a readme or a readme without a clause about their content try to contact them and ask them personally for permission.
If you can't contact them, don't use anything!
If you have permission to use content and you use it, give the original author credit in your readme!
Btw. even if their readme says you're allowed to use their stuff it's polite to also drop them a note. Not so much to ask for permission but just to tell them their stuff is going to get used and maybe a thank you. Knowing their work is appreciated makes everybody happy.
You can use everything you find in
my maps for KF projects. Don't copy whole chunks of the levels (i.e. don't just make a lights-out version), but feel free to take models, textures, sounds, brushes, emitters, etc.
If you need textures cgtextures.com is an excellent site with loads of quality textures and very lenient licensing.