Do you think as a start up blogger I must limit my post to less than 1000 words for SEO benefits? (Dose content length really matter?)
Don't limit your posts except to eliminate unnecessary words. This is important whether you write 100 words or 1000 words. Remove words or phrases that won't change the meaning of the sentence.
Instead of "I didn't eat anything at all" you can say "I didn't eat anything,"
Is shorter 500 word blog posts also considered too much (I know quality matters. I am asking from search engine point of view)
You can't focus on such details.. they're meaningless when you take other factors in. Steve Pavlina's blog has very long posts with some terrible titles ('motivation') but he's generating 10k/day+ - and there are people with fully optimized sites that generate a few pennies a day at best. Like every other reply said: Focus on content or hire somebody that will.
What is the approximate keyword density I must maintain for each of my posts?
I've always heard 3-4 but I personally can't reach that without making the article almost unreadable and spammy. At least 1 is ok..
Are the keywords in my domain name hugely important? (Canít find any good domain names on my niche)
Technically it has a big impact on your rank but is it important? probably not as much as a brandable/easy to type and remember domain.
Do you think I must post less attractive topic at first and then after building some readership go on to write some solid articles?
Write 10 articles and proofread them, but don't publish. Then just have 1 publish everyday.
Most of the articles I plan to write are already covered by websites like entrepreneur.com and related. So do I stand a chance for ranking for them?
It depends how much time, money and effort you're willing to invest... most people would say 'never' but it's feasible.
Do you think I have a chance of success, now that there are already many, many, many established business news websites?
I think your chance of success is slim just because you're focusing on SEO instead of content. The many successful blogs right now began with a bored dude writing about his hobbies. You also have no leverage (or do you?), i.e., other sites/communities you own or are a part of.
If you wanna succeed, close this window and get to writing. GL!