Windows 8 and newer have two reset options. The first reinstalls the operating system, but leaves your stuff alone. You don't need to reinstall any software and your files are safe. The second option option has three sub-options.
1) Securely erase everything and reinstall operating system.
2) Same as the first option, but without secure erase.
3) Erase system drive only and reinstall operating system. Other partitions are left alone. Meaning you should copy your stuff from C: to another drive if you want to keep them.
No need to use external media for reinstall as long as you don't replace your system drive. But if that does happen, you should be able to use your free Win10 license key to reinstall from external media (DVD/Flash) by downloading image from MS. There should also be no restrictions on what computers you can install your copy of Win10 on, it's the same as if you went to buy a copy of Win10 from a local store. That's the way it worked for Win8 and I don't see any reason it'd be different for Win10.
Also, having used Win10 beta builds for a good while now, I can say that your "Mah Win7 puters will be left bloody alone" comments are ridiculous. Get on with times people. We don't want Win7 to be yet another WinXP. Especially when Win10 is given away for free. You didn't like Win8, that's fine. You have nothing to fear as far as Win10 and desktops are concerned.