Average game size is 30 to 50 points/soulstones. Each model is priced using a soulstone cost, not points like in Warmachine/40k. There are no "units," but many models come in a set of 3, so its like having awesome solos doing more awesome stuff. You can play Malifaux right out of the box, which for my most recent purchase, is at 27 points and has 6 models.
I don't build my list based on what my opponent is using. I believe there is actual section in the rulebook that determines when your list is actually made, after the size of the game has been decided, and then you "hire" your crew as appropriate. Scenarios/deployment/other before-game-starts are determine by what card(s) you or your opponent flip. There are no dice in Malifaux, except as a turn counter, so the games are quick (only 5 turns, with additional rules for longer games).
I don't own a significant portion of the 2 factions I play, and any new models I choose to buy are based on what I already own and how that new model can support my crew(s) and its play style.
I've seen most tourneys played at 50 points (max). Some are probably smaller, depending on the tourney organizer. I own far more than 50 points of models for a particular themed crew, but it gives me variety on what I want to do and how I want to play it.
I find 2E more balanced and streamlined than when the game originally came out several years ago. I didn't understand the rules well enough and the models were complicated and the stat cards were a disaster to read (personally).
And being someone who has played Warmachine for several years, Malifaux is a nice way to get away from huge games and lots of models. It's simple and streamlined.
I hope I managed to answer some of your questions.