Big topic but low search traffic? Good niche?

Hi Guys, I'm new here. Thanks for coming to have a read of my quandary.

I'm looking to setup a passive income stream using all the techniques that I've been learning in SEO and web design. At the moment I'm exploring a few different possibilities.


Cutting to the chase...
I've noticed that on a number of different forums on the web there is one topic that comes up again and again, lots of people talking about it, lots of people asking the same questions over and over... and the answers are at times confusing and inconclusive. It's a serious issue that affects people's lives and the answers to those questions (if answered by a qualified person) could really help people out. People even pay an solicitor to answer these questions for them and guide them on how to proceed, so its big business (consultation rate around £1,000-£2,000 per person).


What's the idea?
Working with an solicitor who specialises in this area, I want to setup a site which collates all the answers to these questions and conclusively answers them as the solicitor would, but without the heavy price tag. I plan to give this information away for free, and work my way up the search engine rankings (which are quite competitive because of the forums).
I also plan to monetize the website with premium content, ebooks or potentially even a membership Q&A service.


So... what's the problem?
The problem is that I've some keyword research and the particular questions that people are asking on forums return incredibly low search volumes on Google.


My questions to you
Howcome if a topic is so popular on forums, it's showing low volume on google adwords keyword planner?

Should I build the site anyway and promote it on the forums as "the expert answer to your question can be found here [link]"?
- In this way I'm not really relying on search engine traffic at all, but instead being the "go to site" on forums for having these questions answered and collated into an easy to understand step by step process
- I hope to get other people to link to my site on forums in this way, using my resources to help other people

Or is this not a good idea because the search volumes are too low, and the keywords are already dominated?
- Lots of gov sites with high DA and PA on searches with any volume
- Or could it be that the long tail searches will give me enough traffic to make this worth while? (Maybe keyword planner doesnt register them because the volume is so low).


Thank you!
Thanks for reading and thanks in advance for any replies.
Tom