Photography and image based sectors seem to work well in G communities. There are some very large IT communities but they tend to be mostly people either wanting help or advertising their own services.
I see some communities where the people involved are people I would consider customers. So yes this alone makes it worth consideration. Personally I think it is a good way to meet or connect with other people in your industry.
Benefit to rankings? Well all the G+ links to your profile carry page rank...and then the G+ ecosystem also has its own rules for Follow and no follow links ...so yes there is potential here for the communities and profiles to be treated and ranked just the same as any other http page.
You have to have a G account but this is easy to do.
Yes...you create a brand or business page as a subset of the individual account.
If you retire? Well the simplest option is to appoint your successor as a moderator to help monitor and control the community.i don't know if there is a way, at the moment , to change the ownership of a community.
Company details? Well yes they are changeable but for communities I am not sure that it has relevance. The community name will usually be the industry in which you operate plus maybe other specifiers. Very much like this forum.
The easiest way to explain this will be through email so I can send the links ..PM me if you like. I am just a beginner at this,there are better qualified people and I can direct you to them...see Mark Traphagen, earlier poster , for instance.
But I can show you what I do know.
Sector is travel uk based. I do however have a small understanding of some heavy industry sectors...power stations, industrial filters and so on.