I wouldn't bother getting Sonar 8.5.
I just reinstalled my old versions of Sonar 4 and Studio 8.5, just in case SPLAT goes down and I need to fire up a project, and I can tell you - it's a labour to use after the smoothness of SPLAT. A real step back.
I agree with Cactus Music, get Mixcraft - it's newer, smoother and will be updated and de-bugged going forward.
Sonar Studio 8 was sold as a CDR version - 8.5 was only available as a download if you already owned Studio 8 (it doesn't work without Studio 8 being installed), and all the updates and fixes (3-4 if i remember) will not be made available to you by Cakewalk now.
Waste of your time and money. You'd be better off upgrading to a newer computer.
Also, XP software works in Windows 10. There is a compatability mode - I'm still using old software from pre 2000 with few bugs.