I wanted to buy my first two lieutenant packs for core set and after some reading I choose Splig and Belthir .

Is it a good choice? I want to have just two of them for now.

They all rock, imo though I like Sir Alric. He is a bit of an all rounder with monster buffs and is a powerful agent on the board. Overrun can really break up the heroes position. His plot cards can be purchased based on needs and the flow of the game.

Valyndra has a power hitting deck as well.

In stating those facts, I like Splig for the fun factor. Belthir is a little less favorable because he constrains me to use Basic I.

I like lady Eliza Farrow's abilities and as I only wanted to buy into one or two lieutenants ... is her Plot Deck any good?

Some seem to be quite specific...

Eliza's was the first plot deck I've used, and it's not bad. It's certainly not the best of the plot decks, but there are some pretty nice cards in the set (like Nighttime Hunt or Bad Dreams).