Best way to get Traffic Estimates & Keyword Difficulty for Long Tail Keywords

Does anyone know of a better way to get accurate estimates of long tail keywords and even keyword difficulty?

Currently, I use Google Keyword Planner to get search volume for keywords and then use the MOZ toolbar to get keyword difficulty. I also judge keyword difficulty just on results from a search and use Keyword Planner as well as Google Suggests for new keyword ideas.

The problem I'm having now is getting an accurate estimate for long tail keywords. For example, some words when searched may display an "-" in Google Keyword Planner to represent no search volume. However, this doesn't seem accurate and it's hard to tell for long tail keywords like this if they will bring in traffic.

Does anyone have a better way to find accurate data for long tail keywords?

As far as keyword difficulty, what's the best way to judge if a word will be easier to rank for? Currently, I base this decision off MOZ Keyword Difficulty results and intuitively from looking at search results. Are there any other recommended methods or tools to use to for this?