What is important for PS (including Yandex):
1) Valid code (i.e. the absence of gross errors, clear structure, no traces of the "bad" optimization — cloaking, "point" links and other twists);
2) the uniqueness of the content, the degree of "pereaminirovanii";
It's in the first place.
My experience tells me that no matter what the cms were taken as a basis. It is important that You continue to do with your site — will increase the trust from time to time to write good. the content to help the SS index and examine your website (xml site map, directories of PS, adding in the panel of webmasters, proper setup of the server to transmit the proper headers, etc.)
The tops have blogs on Wordpress, Drupal... on a par with commercial CMS.
BUT of course, that any free CMS it will be necessary "dopilivat". At least plugins to accelerate and optimize work with DB)
By the way, buy a commercial system does not guarantee that the code will be super good and proper.
+ Each project has its own specifics. The blog with the attendance of 250 uniques a day does not have to go for it.
But the Internet store you should think about the loading speed of their pages, I think no need to explain why)