For selling miniatures, on the sprue is fine. Assembled probably lowers your resale value a bit, as the buyer now can't put them together they want. Painted takes that a step further, unless the buyer wants to paint them the same colour you have, its more work for them. Pro painted models can sell well, but that's not likely something you need to worry about for a while :)
Don't bother buying GWs brushes and modeling tools. They're overpriced, from all accounts. I haven't needed to buy anything in years, so not sure exactly what they're selling for. I just bought my tools from a hardware store .. a pair of snips, some small files, a small hacksaw and craft knife. Brushes you can get from any art / craft store.
Paints .. there are a variety of different model paints available. Have a look at this for a few different ones.http://www.dakkadakka.com/wiki/en/Paint_Range_Compatibility_Chart
In terms of essentials to play ...
Basic box set - it's pretty good value for money, and will give you the starter of an army, plus the core rules
Codex - needed for the army you want to play
Tape measure and a handfull of dice - Not too hard to get hold of.
Pair of snips and/or craft knife for removing models from sprues / chopping models up for conversions, small file
Paintbrush and maybe half a dozen paints. Undercoat (I normally use black), base colour(s) of model, weapons, base model stands on
For example, I'm currently starting a Night Lords force. What I'll need for that would be black for the undercoat, blue for the armour, red for helmet eye pieces and wings, bone for skulls and terror markings, silver for weapons, probably green for the base the model will stand on. That'd be about normal for a single model.
If you do decide to pick it up and want to trade off any of the Chaos stuff, let me know :)