Is the buff fem shep mod worth getting and what other mods are worth getting?

Take a look at this thread:
ME3 suggested mods?

As for the specific questions:
The 'mod' that makes the game harder - I believe you mean the Respawn mod by Sil. (There now is an additional mod that started as patch to respawn, I just can't remember the name) It is pretty great, even mixing up the enemy units encountered in specific missions, based on what your Shepard did in previous games. But the number of enemies it throws at you can be overwhelming, especially for weapon centring shepards that use weapons with low reserve ammo.

As for the Saber - Believe there are several mods that let one acquire weapons earlier or differently. I personally use a coalesced edit of my own that adds weapons to specific stores. It's not hard to make and so far has been compatible with the DLC mods that I use. (It has a few drawbacks, though)

I never tried the buff femshep mod, so no idea there.