Klaas not sure anyone can tell you what the magic number is as far as being too competitive or if it's a good niche to target. It depends on so many things. Maybe someone will give you their opinion but those numbers may seem like a piece of cake for one person and too tough for another.
Reading about KEI may help you somewhat, but I've yet to see anyone say "look for a phrase that has "this many" competing pages.
KEi Keyword Effectiveness Index - Google Search