>Can you make changes in Archicad and reference back to your C4D project?
There is a C4D<->Archicad exchange plugin that allows you to go make changes in Archicad and update in Cinema. I haven't used it, personally, so I would check Maxon's site for more details.
>Where is the best place to go for architectural tutorials?
I don't know of any Cinema-specific site, but evermotion.org has a lot of Archviz stuff, and the same goes for cgarchitect.com
>Does C4D use network rendering?
Yes, depending on which version you buy. The base package doesn't have net rendering. Broadcast and Visualize come with three render nodes and Studio has unlimited network rendering.
>Do you find C4D expensive to maintain, upgrades, models etc?
It can be pricey -- in line with the other major 3D packages.
>What would I need in terms of C4D versions, plugins etc for
Cinema comes in four flavors -- Prime, Broadcast, Visualize, and Studio. The natural choice would be Visualize which comes with all of the rendering bells and whistles. Unfortunately it does not come with the Mograph module which is quite useful for architectural stuff. Broadcast comes with Mograph but it's missing some of the rendering features (most notably, photometric lights, depth of field effect and Sketch & Toon (non-photorealistic renderer)). Studio comes with everything, but of course it's more expensive....
So it depends a bit on your priorities and your budget.
In terms of third-party products, I know that a lot of archviz users are using Vray4C4D which plugs the VRay render engine into Cinema.