What instruments and samples/soundfonts or whatever do I need to make a realistic string or brass sound?

Free or commercial?

Sonic Synth has spot on Brass and String ensemble's in it.... it uses SampleTank as a VSTi player.

If you go to http://www.sampletank.com/Main.html?STFreeDwn and download their Free player, there are some example sounds for you to try. You can see if the quality is ok then buy Sonic Synth for about $199 depending on where you get it from. I think SampleTank L goes for about the same price. Both are excellent and have good broad ranges of all the bread 'n' butter sounds you're likely to need.

For free stuff I'd head over to http://www.hammersound.net/index.html - they have tons of free Soundfonts. As for a player, that depends on what system you're using. I use FL Studio and the built in SF2 player does me fine. For something a little more exotic, you might want to try http://www28.brinkster.com/experimentfour/x4_fntmntr.html. The Fontaminator is a SynthEdit VSTi which allows you to play SF2's but also mangle them beyond recognition.

Hope that helps

In terms of strings and brass, I've gotten the most bang for the buck from these two sample CDs:

Phantom Horns:
http://www.soundsonline.com/sophtml/details.phtml?sku=TASCD-61M

Symphony Strings:
http://www.musiciansfriend.com/srs7/sid=030329051251140160012052444722 /search/g=home/detail/base_id/60277

Best of luck....

http://kontakt.daw-mac.com/orchestral.html

very very nice stringz here for free

http://www.esoundz.com/viewDetails.asp?ProductID=87


is fantastic!