I am like 90% sure of this, but I want to make sure. Can I add my old campaigns from years ago to the google analytics, so I can show someone interested in buying my website my stats. If not, what's the best way to go about it.
Google analytics show all your traffic and you can also get report where will be displayed only visitors from particular source like the old campaign. In your analytics change the date range to get historical data.
So even If I didnt set any of my sites up for analytics, all my sites will show up with detailed stats?
No. You will not have stats for your site before you setup Analytics. Without the Analytics code in place it can not track your traffic.
There are stats in the Adsense section. These stats you will see no matter if you are using analytics or not. But analytics can give you a lot of useful information about your traffic and can tell you what works. So I would recommend to use it.
Google will ask you to place a code on your website. Once verified, it will start analytics. Did you add the code to your website one year back ? if yes then you can directly go in analytics and take a print.