Is it essential to have a validated website to reign in the search engines? Yes and No. If you are buying a website it is a good idea to ask for it to validate before it goes live. For the simple reason that you can then be assured that your site meets certain standards and will not contain any problems which could get in the way of the sites SEO chances in the future. If your site validates then there’s a good chance that there is no on page javscript to confuse Googles crawlers or iframes in which your precious content could be hiding. In the same way that an M.O.T. Can give you a reassurance that your car meets a certain standard without having to know anything about mechanics, a validated site can give you certain reassurances without being code savvy.
On it’s own Validation will not make a huge difference to your sites rankings but it ensures that your code is easily readable by a search engine and that’s an advantage in itself. If your site is live for a couple of years before being optimised then it could be held back by all kinds of troubled code getting in the way. A quick check of the top ten sites for a competitive keyword will show that validation isn’t essential to rank well and two or three validation errors aren’t going to stop your code being read. However its a good place to begin when starting an optimisation campaign and it gives website buyers the knowledge that under the hood of their website it’s not a piece of junk.