Yeah as above, if you use XF1 and start adding styles and addons and customising your site to your needs you'll have to start all over again when you upgrade to XF2
Yes @CalebPrime4 , but XF1 is no longer supported with updates as it's reached it EOL (End of Life) so any security issues that may arise with XF1 will not be patched anymore, so it's recommended to simply just use XF2 over XF1.
Yea. An active license can download any version of xenForo. Once it expires, you'll have to renew for future updates but you can still use any old version of xenForo. Take note that xenForo 1.5 is no longer officially supported here.