As for which devices are going to be the first to get the Android 5.0 – Lollipop update.
Nexus 4, 5, 7, 10 devices will get the update early doors. There is no mention that I can see on which generations of these devices will get the update, perhaps all.
Nexus 6 and 9 which have just been released will ship with Android 5.0 – Lollipop.
Motorola, Moto G, E and X will receive Android 5.0 – Lollipop via OTA shortly after the release, exact releae schedule will depend on your carrier.
HTC One M7 & M8 are scheduled to receive this update within 90 days.
Android One phones (Micromax Canvas A1, Karbonn Sparkle V, Spice Dream Uno) will also see this update.
Sony are saying any Z-series Xperia device will receive the update, no word on exactly when though. Here is the list: Xperia Z, Xperia Z1, Xperia Z1S, Xperia Z Ultra, Xperia Z1 Compact, Xperia Z2, Xperia Z3, Xperia Z3v, Xperia Z3 Compact, Xperia ZL, Xperia ZR, Xperia Tablet Z, Xperia Z2 Tablet and the Xperia Z3 Tablet Compact.
Other manufacturers like Samsung, LG, etc are being tight lipped at the moment. They will likely spend some time integrating Android 5.0 – Lollipop with their own customisations and release it for their latest phones only to try to get people to upgrade.
The fact that Android 5.0 – Lollipop is on the Motorola Moto E and Android One devices should mean that it can run on relatively low spec devices.