If you're going to stick with the display, there's no need to upgrade the CPU/MB/RAM/GPU, they're all more than capable of running that resolution, I suspect, even with modern games pretty well maxed out.
If you want to boot off the new SSD be aware it'll be tricky to migrate the OS to it, probably the best option would be to bite the bullet and reinstall everything: OS, games and apps taking the opportunity to update ALL the system software while you're at it.
Me, I'd go for a 2Tb HDD and a 256Gb SSD.
Suggestion:
Get a large USB drive and download, then save, the latest system software to it, that way you'll have the latest software for the system and peripherals to hand.
The same drive can be used to hold your valuable data, so it can be imported to the fresh Windows install without loss.
You can also use the Windows Media Creation Tool to download the latest Win 10 to the USB drive, assuming your system has USB 3.0 installing this way is much faster than using a DVD drive. Your existing key will activate the new install, just select 'I don't have a key' during the install process, Windows should automatically activate using your existing key or through the 'digital license'.