Half-Life is by far the easiest game to make mods for, Unreal is the most powerful engine around, but it is also the hardest to use.
For making Half-Life TCs you would need
PakExplorer
Wally ( for .Wads )
Milkshape 3d ( for models )
Worldcraft ( for levels )
and that's the basic kit. of course there are alternatives.
But, thats the basics.