A reasonable choice back then, but a better choice is to use the long term support releases, which are supported for several years. So back then it would have been better to choose 16.04, now it would be better to choose 18.04.
When an Ubuntu release reaches its “end of life” it receives no further maintenance updates, including critical security upgrades. It's highly recommended that you upgrade to a recent version of Ubuntu at this point.
17.04 is already beyond end of life. Such releases only get 9 months of support.
It's not just about the length of support. A new version comes out every 6 months and although it's fairly solid they're exploring new possibilities, whereas the long term support releases, every 2 years, nn.04 (where nn is an even number, corresponding to the year, and 04 is April), they know they have to support it for 3-5 years and so it's as stable as they can make it.
So 18.04 is the one to go for. And will continue to be so, even after the release of 18.10, 19.04, 19.10. When 20.04 comes out you wait a few months for it to settle down, then upgrade.
Or if you're like my wife, even then you don't upgrade! She was on 12.04 and continued with it despite 14.04 and 16.04 coming out. Only when the 5 years were up did she move to 16.04, and that's what she's still on!
Lubuntu (Ubuntu with the LXDE desktop, less resource hungry) should run fine in 2GB.
Always good to have backups, for anyone.
Not necessary. Advisable? Up to you. Might be good to clear the cruft out of Windows, but that's not relevant for a Linux install.
If your 256GB USB has been delivered formatted (and I've never heard of them not being so), that should be fine for your backup provided the total storage that your using on your hard drive is less than 256GB. You make the backup of your Windows system, when it's finished you use 'Safe eject' to ensure everything is written to the USB, you wait to be told it's safe, then you take out the USB stick and store it in a safe place. It plays no further part in your Linux journey, but if something goes wrong, you can fetch it from its safe place and use it to roll back to your world as it was.
You didn't answer my question: You need to tell us more about how you did that backup.
Without an answer to that we're just making wild guesses about why the backup may have failed.
Alvays anzer zer qvestions, jah?
Very best wishes, let's make this the summer of progress!