Looks like a solid list and you've found the allied system with the Cannon! The only downside to Dispossessed is that they have no Battletome, that said considering how many models they've got in plastic I'd wager that they are fairly likely to get one of their own (it might even bring the Cannons and such back into the faction as right now they are split off).
Like I said a fair few factions can struggle at 500 points with regard to options and diversity; that gives you a nice cheap core of troops, a leader and some ranged support. The warriors are an older sculpt so might one day be replaced sooner than some others, but honestly I doubt that will be for many years as yet.
I'd say its a good start to a halfsized army!