Actually, I'm doing something similar to the original poster, so perhaps my experiences and observations will help.
I started with my SO will all my available cards, but that was too much to absorb, so we went back to the core set only -- trying the 4 main factions for a few games until my SO was comfortable with the faction strengths and weaknesses and develop favorites (She prefers chaos, while I prefer dwarves, though in replaying some things, I think I've been overlooking empire too much.) Next, we added in the first cycle of cards (corruption), actually starting to build decks -- me with a dwarf deck and her with an empire deck. Our next step is to add in the Assault on Ulthuan and play with the elves a bit, especially since she likes dark elves. (I only like light elves as secondary support and healing for a different primary faction.) We plan on adding things chronologically since I only own a couple of cycles currently. I don't plan on buying any more cards until we've added in all the cards I own and we're both still interested in playing.
Advantages: You don't need to deal with every card that currently exists -- you only deal with the game as it was at a given frozen moment in time. As long as you don't play against more modern decks, that's fine. Yes, there are shinier, newer cards, but you don't need to sink more money into the game if you're not competing against them. As for balance, the balance was fairly close -- elves were a bit weak as main factions initially, but the other factions are relatively balanced (we're also doing this with Cthulhu, which feels less balanced, relatively speaking, and taking a random selection of packs is less balanced). As for the elves, introducing the elves as support factions works pretty well if you're trying to introduce things slowly. In short, the advantage is not being overwhelmed by the possibilities and getting to know the cards slowly.
Disadvantages: You won't be competitive against outside, modern decks. You won't be aware of new mechanics, like legends. You won't have access to the shiniest or most powerful cards. However, if you're just playing from a closed set collection, this really doesn't matter. You're just experiencing the meta at previous points in time.
In short, while it is possible to keep costs down by building decks from a database and just buying the packs that contain the cards you used, that can be overwhelming for new and casual players. While that is better for getting into tournaments quickly and cheaply, I feel that my method is better for casual play and introducing the game to a new player you hope to get hooked, if you want to minimize the feeling of being overwhelmed.