Step 1 invest in keyword research so that you can rank in google quickly (relatively - Google takes its time these days)

Step 2 invest in on site optimization - many sites used to adwords are not properly optimized in title,e H1 tags , navigational architecture etc (not hard or expensive).

Step 3 invest in building strong organic links

and yes you should be doing some SEO. Every visitor that comes to your site for free increases your profit and some searchers will never click on adwords at all.

thank you for the fast reply! Keyword analytic & onsite optimization is already done. We got an 100% custom coded website & optimized texts with perfect html markup for service lists, titles, page links etc.

So the most important point are backlinks, articles etc to get traffic & rank

Keywords will always be important if you're focusing on SEO. Personally, I find it more successful to use PPC through Adwords or Facebook rather than struggle to always beat the person ahead of me in PPC cost wars.