What Android version promises changes with regards to fast security patches? I've heard that an upcoming, or recent version of Android has separated the OS from the hardware specific parts, promising much faster security and minor updates for the user.

What version is this?  And are the changes so significant that a buyer should consider waiting until that version is available in phones?
From what you described you probably meant Project Treble.


  The Android 8.0 release includes Project Treble, a major re-architect of the Android OS framework designed to make it easier, faster, and less costly for manufacturers to update devices to a new version of Android. Treble is for all new devices launching with Android 8.0 and beyond.


Per the above quote, any device shipping with Android 8.0 or above (Oreo) must support Treble. However, devices shipping with a lower Android version and subsequently updated to Oreo are not mandated to support Treble.

As the launch of Android P draws near, almost all newly released devices ship with Oreo, so you don't have to take the factor into consideration; however, if you're eyeing older devices, then be sure to research whether it ships with Oreo, and if not, whether it supports Treble after updating to Oreo.

As for how significant Treble is, personally I'd say not so much. Just because it's easier to do doesn't mean that manufacturers are inclined to do it, especially for lower-end devices that don't bring forth good margin. Also, manufacturers still have to take care of their own skins and additions; carriers still take their time to review updates (and add bloat?) before pushing them out.