When it comes to seeking funding from VC's or Angels - if you actually think that's a viable form of funding, quite frankly you need to rethink your financing strategy. Sorry to be blunt, but in my line of work (I'm the co-founder of an emerging business incubator and my co-founder runs another emerging tech incubator that I work closely with) that's the biggest mistake assumed by many of the companies I encounter. Typically only 2% of all ventures pitched get funding at all from Angels and VCs.
As far as a niche, typically VCs will seek literally a 15x return minimum on their investment while Angels I believe see a 20x return minimum. As far as funding goes - VCs usually do rounds of $1mil and more - sometimes $500k
Angels - one of my business partners is one, which is how I know this information - usually you can go as low as $5k to as much as $1 million - the key difference between an Angel and VC is that an Angel (as the name implies) invests to help others or they do it as a side investment whereas VCs simply care about the money and the like.
As far as a minimum audience, that's really hard to define. Quite honestly I've found from my experiences that if you have a good concept and a good execution strategy, you can get funding provided there's a reasonable crowd. The definition of "reasonable crowd" varies greatly from industry to industry - i.e. a high end car might have a smaller niche but you'll have a higher profit margin. On the other hand, a mass produced product will need a much larger market due to the lower margins.
My best advice to you for funding however is to really try starting small and scale accordingly. You can't go to VCs and Angels without a proven concept regardless - so you will likely either need to take out a loan or bootstrap. Once you have your prototypes and financial figures in order, you'll be in a position to pitch.