Sorry if you already know some of this, but I'll start with some basics.
1. iCloud is a collection of services to allow syncing and access of files between multiple devices (contacts, calendars, docs, etc) and backups of your devices.
2. iCloud Photo Library is just one of the services of iCloud. You can turn it ON or OFF on any particular device and use manual management, via iTunes, or My Photo Sharing.
3. By definition, iCloud Photo Library contains ALL your images and videos at full resolution (in effect, it's your "master" library).
4. It is NOT a good idea to keep your originals only in the cloud. You should also have, in effect, a backup on your computer and in whatever photo app you're using. If you're low on space on your computer, invest in an external hard drive - they're very cheap now.
5. To conserve storage space on any device, you can select in Settings/Preferences to keep an "Optimized" library on any of those devices and still retain the feature of having access to all files. With this option, PC and/or the individual device will automatically and dynamically use only as much storage space as is available, and will free additional space if you need it for other files. It does this by storing locally small thumbnails of older, or less accessed images, instead of at full resolution.
6. When iCloud Photo Library is enabled, you no longer need to manually move your images from devices to the library - it all happens automatically. If you're using Photos app, edits to your images are also synced.
7. iCloud Photo Library will use up a lot of data for the first sync. Set your devices to only use Wi-Fi, not Cellular, data.
My suggestion for what would work best for you:
A. Set your computer to use iCloud Photo Library at full resolution;
B. Set your iDevices to use iCloud Photo Library, but at Optimized resolution.
C. Continue to sync the second device manually via iTunes.
You might find this article about iCloud Photo Library helpful. Otherwise, search this forum for specific issues or questions.
http://www.imore.com/how-use-icloud-photo-library-ultimate-guide
PS: I'm on Mac, so I might be making some incorrect assumptions about what can and can't be done on the PC side, but my iOS instructions are solid. There are seperate forums here for Windows users that you might want to visit.
PPS: To answer your specific question about the "deleting" of images when you turn iCloud Photo Library... Only the local Photos library on the iPhone will be deleted; the library on iCloud will not be affected. And you stil have/should have everything on your computer, anyway, right? See item #4 above.