I am planning to customize magento design from scratch, for that i have investigated and found many solutions, some with bootstrap and some without bootstrap. I just wanted to confirm what is the best practice for this, should i invest my time mixing bootstrap or should i just edit existing magento css file.
If you need apply some changes maybe the best way is to start here: http://devdocs.magento.com/guides/v2.2/frontend-dev-guide/css-guide/css_quick_guide_approach.html
Adding Bootstrap from scratch will demand a lot of work because you'll need to apply all the styles to the whole store (I guess).
(my 2 cents)
If you were going to re-use your theme for multiple websites it may be worth putting in the time building and integrating bootstrap if that's what you are familiar with and prefer. From experience it would be quicker to take the existing Magento 2 theme and go from there.
Another option could be using a bootstrap theme that someone else has created. There are a few out there just quickly looking from Google. https://www.google.co.uk/search?q=magento2+theme+bootstrap&oq=magento2+theme+bootstrap&aqs=chrome..6...