Error 503 is a server error at their end, nothing to do with you or your country of residence. 
 

You can set up an account an buy a BahnCard from outside Germany, so I guess try later, or with a different browser. 

I guess you've made all sorts of calculations but in my opinion the best deal is :

- Interrail Global Pass + Eurostar reservations to get to Berlin

- Deutschlandticket : 49€ for a month, all regional trains and city transport included (you might not even need it if your focus is on Berlin)

Thanks for the suggestion but that’s too expensive for a simple out & back trip.

It heavily depends on your travel date since Eurostar can be well over 150€ per person while the reservation has a fixed 30€ price (though when 170€ tickets are on sale the passholder quota runs out). 

Of course bought in advance (esp. on weekdays) a Eurostar + ICE ticket on bahn.com may be cheaper.