I ran envisalink on my main hub for 2 years before I split it off. I didn't want to bother with the whole rPi thing. So I'd say start there and see how it goes. A second HE is pretty inexpensive if it becomes an issue down the road.
Yes with the Vista panel. I thought I read somewhere that one of the panels only sent events when the panel was armed, which makes them less useful. But in my case they all act as virtual sensors and are available to all HE apps. Really quite handy TBH.
