These tools are kind of a must before & while managing any SEO project(s),
Ahrefs, Semrush & Google ad planner are the best in my point of view,
If you need something cheap, you can go for ad planner (free) and Semrush (30 days trial with BHW custom deal, just using $5 VCC),
Such tools can be really useful to filter some list of keywords for search volumes, but you can't totally rely on them for difficulty as you'll need a custom SERP analysis to get what might be needed to rank for some custom set of keywords