hello Peter,
The modules installed on DotNetNuke.com are the free core modules.
I can't really offer any advice on moving over from Xoops CMS as I haven't used that system.
You may find however the free QuickStart guide videos helpful:
An Introduction to the Principles of DotNetNuke
Quick Start Guide to DotNetNuke
There are no forks in DotNetNuke, and you are correct that the core code is maintained by official DotNetNuke staff which helps to prevent some of the problems you have experienced.
You should also find that the modules available for purchase keep up-to-date with the latest versions of DotNetNuke and most of them have a good support network.
The first thing I would do is watch the quick start guide videos and then install DotNetNuke on a local host installation and have a good play around with all of the settings.
Hope this helps to get you started,
Thanks,