[rl3]

>Also, what percent of a startups (let's say app company) success is reliant on marketing?

This really depends on the vertical. Generally speaking, the less reliant on paid marketing you are, the better. Truly excellent apps market themselves. If you're entering into a niche vertical however, having to deal with marketing will probably be an unavoidable fact of life.

As far as competitors, it's almost always a bad idea to fret over the competition even if they're established. In this case they clearly aren't, so your only worry is whether to take heed of the fact that all of your potential competitors are basically dead on arrival.

As the other comments say, this really boils down to your judgement. Check out the other apps yourself, see how similar they are to your own execution ideas. If they're identical, you may have a problem.