Hmmm… You don’t say exactly what you want to do so you’ll get only vague answers.
To make various texts appear the one after the other, you can just keyframe the renderability in the Outliner, and the visibility too to make things clearer. (Layer animation has been “fixed”… i.e. it doesn’t work any more.)
For your example, on frame 1, you set “Hello” as renderable and “World” as not renderable. You keyframe the 2 states. On frame 50, you inverse the renderability of the 2 texts, set the 2 keyframes. Then at frame 100, you set “World” as non-renderable. Animation done!
If you want to get fancy or instead of animating the renderability, you can animate the materials thanks to a mixer node to choose in between the “real” material and a fully transparent one. Animating the mix factor allows you to have tweakable fade in/fade out effects… which can as well be instantaneous and produce the same effect than animating the renderability.
Any way, I would always consider animating the renderability first. Less objects means faster rendering. Saving a few ms per frame turns quickly into seconds then into minutes… eventually into hours.
As for the 125 text objects, I can’t say much. I don’t know what you want to achieve.
Last, I think you may want to look at RE:Lay and RE:Phrase which might interest you: http://www.blendernation.com/2011/06/06/relay-and-rephrase-add-on-blender-2-5/