...or the happy weekend ticket (which sounds like the cheaper option so perhaps we should go on sunday the 26th)
Exactly, Weekend ticket is cheaper + it is valid from midnight and not 9am as quer-durs-land, thus will allow you to travel from early morning.
Where does the german group ticket take us to (i.e. what is the border station that is the last valid station?)
This is almost an open question, since there is a lot of border crossings between Germany and Netherlands, thus you actually can choose any. But counting domestic Dutch part (for which you will need to pay full one-way fare), makes sense to make it as short as possible. Geographically it will be area near Nijmegen, thus German part will end for you around Kleve (->Emmerich/Kranenburg). But the problem here is that there is no good connection over the border there, just bus. Thus it might make sense to go via Gronau->Enschede or Kaldenkirchen->Venlo. Both Berlin-Gronau and Berlin-Kaldenkirchen will take app. 9-10 hours with 4-5 changes. Fare and timing from Venlo or Enschede to Amsterdam are also similar - around 20 EUR and app. 2 hours.
Another tricky option here is to travel till last big city before the border (like Duisburg, Berlin-Duisburg with Schönes-Wochenende-Ticket will take app. 8 hour and at least 3 changes) and purchase from there discounted ticket for direct ICE till Amsterdam, for your date there are still ticket for 24 EUR available (which is almost the fare for domestic Venlo/Enschede-Amsterdam anyway).
and where to we purchase the ticket from there to amsterdam? - could we get it on the train?
In Netherlands it is not possible to purchase tickets on the train, thus you will have to manage without this option. More about purchase options here -
http://www.ns.nl/en/arrange-and-buy/tickets-and-passes/purchasing-tickets which train lines are we allowed to travel on with a german group ticket?
Actually any where DB regional trains run, the limit is only on train types, thus you can't use high-speed ICE or IC trains, as well as CNL, EN/EC. That is why you'll need 4-5 changes in order to get till the border with Netherlands - regional trains don't run for a long-distance legs.