I would say definitely add it:
A DOCTYPE (document type declaration) tells the browser what particular version of HTML you are using to code your pages. More can be reading here:
Document Type Declaration - Wikipedia, the free encyclopedia
Looking at your coding which is a mix of stylesheets and old HTML formatting, I would say try transitional rather than loose.
But don't be put off by 74 errors, although clean coding is a step in the right direction, it is debatable that it has any effect on ranking. it is better to concentrate on more key areas such as tweaking your content to reflect your key phrases and starting a link campaign.
A good article by Incredible Help can be found on this forum:
http://www.webproworld.com/search-en...o-i-begin.html
Hope this helps.