Hey! I'm going to create a big site (not a mini site) around a popular topic. Let's say (only for example) that the first of the primiary keywords is -digginggold it has 70 000 exact match searches each month the digginggold dot com, net, org etc. are all taken, and its rather competitive term. The second keyword is lets say (again for example) - digginggoldguides it has got 15 000 exact match searches each month digginggoldguides dot com is taken already, although there is no site, only wordpress installed at the domain. ORG and NET are free to take. Should i take digginggoldguides dot net or org? I think this might cause some auto traffic for my competition later on, when my site will be the most popular on this topic. If yes, which one? Or, maybe I should get less keyword rich .com domain and just build up a brand around it?
In my opinion, you can go for less kw rich domain for .com because .com is a top level domain and it will give high reputation to your brand or product. You can create .net with kw rich domain for promoting your .com.
Won't I have problems later with the exact match domains on my main keyword (digginggoldguides)? because from my research this keyword will be the biggest traffic driver for my site.
The exact match isn't really that important if you're building an authority site as you say you are. It would give you a bit of an seo boost, but with a large site you will have many keywords and pages and you can't have all the keywords in the domain name. You shouldn't worry too much about it, rather get on and build the site and start making links.