This. There's no need to buy any Skylanders new. After 6 months they will all be reduced or sold as buy 2 get 3 free. I have hundreds and spent probably $2 a piece on them.
Not always. There are a few that wind up being pains to find. Trap Team was the worst with the Kaos Trap, Sun Spot, Blackout, Echo, Sunscraper Spire, and Midnight Museum all being relative pains to get for MSRP or less. Superchargers wasn't too bad, with regular Thrillerpede and his vehicle being the hardest to get.
Anyhoo, I'm starting a bit light myself, with just the base game and Kaos (who is still in the mail for some odd reason). But, considering that I have a standing army of around 150 of them (not counting series 2/3 or repaints, only missing 8 skylanders across all the previous games), I think I'll be fine.
Still, I'll be getting achievement solutions up on True Achievements as soon as I can, just like with the others.