Most of visitors through adwords keep coming as long as your ads is shown. If you are looking for spontaneous buyers - it is one of the best ways. Also on your seo, keyword relevance depends adwords price
I believe that SEO is the way forward, sure, it takes much longer, not instantly, but I always think long term is better, unless you need instant results...and you are willing to pay for it.
Actually it depends on the niche of your ads and websites means if you have earn online kind of website than you have to place the same niche ads on it than it could be profitable in my opinion.....
Yes actually got and page in google top3 on several keyword, these also got 10/10 quality score on my Adwords, so do you have and top site it also help at your adwords campaign.
certainly it helps! For example, you can find a KeyWord that has a little competition, but a large number of requests! And it will help get your site to top10!