The 30 day restriction refers to how long they are kept in iCloud, not how long they are kept on your devices. The rolling collection of 1000 photos applies to iOS devices, not computers; as long as you enable Automatic Import in iPhoto on a Mac. I don't believe you have to do anything to have them permanently maintained on a PC. I don't have that many in photo stream so I can't test it. When you hit 1000 photos, you might want to copy the oldest to a folder, then add another to your photo stream and see if the oldest photos are maintained on your PC.
One thing to be aware of is that if a photo stream photo is deleted from any of your devices, it is immediately deleted from all of your devices (including computers). To avoid this, copy the photo stream photos to another folder.