Download latest stable version - where?

this is right.
if you want V2.xx you get the 2.1 and than update in-place to latest code.

however, if you starting anew, go with V3

it is overall stable,
it uses the latest kernel and is build on Debian 8(Jessie)
do your research as some plugins where dropped during transition from v2
some plugins that are not dropped officially, may not be fully ported yet.

well, you have to realize that install ISO is not up-to-date. I do not know what is the overall rule about keeping the install ISO current is, but most often than none, especially on the ongoing project, it gets outdated very fast. so yes you should select all in "Update Manager" and update all.
however, on rare occasion the update may go bad, it should not break the system but it will not update anything. so you might need to choose and pick things to update first hand and then run an overall update after that.
for example, if the update all fails, you might want to first select only system related packages, like base, dbus ,lib* etc. and run update on them. and after select the rest and run it separately.
it's rare but may need to be done that way.

I usually do updates until the update tab is empty.

Second, you need to install Extras-plugin. incase you are new to OMV , Extras-plugin is not part of the system, you need to download it manually and upload to plugins manually.

I have attached the file here. just remove the ".TXT" extension from the end. upload it to the plugin tab and install it.
that give you some usefull plugin in addition to what is provided as part of the initial install.
infact most of very useful plugins are provided as part of extras-plugin package.

I am not sure what you mean by " "normal use" of a home user for that thing."

if you are new to OMV please read the guides and FAQ here there is plenty of information in this forums.

ask any specific questions as needed, but search the forum firs.

welcome to OMV

Its best that you install all the updates for the latest security and stability updates. Thats one of the benefits of using OpenMediaVault.

The list can be a bit long, doesnt happen to often, just install and be ensured you got the latest and greatest. These days you want your security updates when you are connected to the internet.

I'm actually happy without automatic software updates, wouldn't want it any other way.

Although a NAS is used at home, most people have quit a few applications installed on it and find it important that it keeps working. This means they want to check their NAS after running any update (except minor security updates maybe). This how companies do it to (test your apps after updating).

I advice against it, but if you really want this, you can create the following task in the taskplanner containing this text:

apt-get update && apt-get dist-upgrade -y