The modern apps option is also in Start8 1.5 I think though it is not defaulting to on.
As for free, I know if you buy Start8 right this second (as of when I wrote this post) you would get Start10 for free, and I know existing Start8 customers can get a discounted upgrade ($3.99 I think)
Start10 does offer specific support for Windows 10 such as a new Windows 10 inspired theme along with a few other tweaks coming soon and obviously is supported on Windows 10 unlike Start8.
Start10 will not work on Windows 8 though, it is for Windows 10 only. Start8 will work on Windows 10, but there may be issues with the new buttons on the taskbar and obviously it is not getting any Windows 10 specific updates.