Properties:
http://wiki.alliedmods.net/Entity_Properties
Messaging players is usually done by sending TextMsg/SayText usermessages.
You can also look at the source code to various SourceMod functions for hints on this. SourceMod is open source under the GNU General Public License:
http://hg.alliedmods.net/sourcemod-central (note, if you do choose to use direct source code from SourceMod, you must obey the terms of the license).
GiveNamedItem (in extensions/sdktools/vnatives.cpp) wraps CBasePlayer::GiveNamedItem
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