Actually because your starting fresh, I would not even touch the old PC. You start deleting stuff and then find out ,,oh oh, where did that go.?
As a rule I never delete much over time. I just pull the old hard drives and put them in a shoe box. The more currant ones I have these external USB drive enclosers you can buy for $20 that I put them in.
But if your keeping your old computer just leave everything in there and it's ready to go if your new one goes out of service ( which I doubt will happen) .
I would think about keeping it as your office computer that way you can keep your DAW off line. That's how I progress through computers, New DAW- old one retired to the office. Office computer don't need to be stellar.
I'm just building a new DAW. My old DAW has 3 hard drives in it. OS / DATA and Backup.
So once the new DAW is set up I'll swap the DATA drive over to the new machine. Everything is on that drive I need to set up my studio. The Backup drive stays with the old one as it has full system back ups specifically for that computer. The new DAW will get a new third Backup drive.
So for me there's nothing to do but swap the data drive, all backups are already done as a daily/weekly routine.
So if you can do this, in other words, if your old computer has a DATA drive, just make sure all your Sonar project files are copied to that drive. You could do the same with an external drive but then it's 2 more moves to copy to and from the 2 computers. For the $50 a SATA 7200 RPM 1TB drive costs, they are cheaper then buying external drives.
By the way, good move to purchase a Studio Cat, I'm building my own just because it's a hobby, but in the end I'll only save a few $$. His prices are excellent for what you get.