I believe, the ferry that had the most modern interior of its time has always been and will always be the famous gas-turbine ferry GTS Finnjet, which was not only one of the most innovative ferries in 1977 but as well hold the title for the biggest and fastest one known for a very long time.
I know some ferries from 70ies, this one had many amenities I did not find on other ones of that time, for example air condition for cabins below cardeck, sauna, variety of restaurants and very colorful and stylish interior. More info can be found here: http://www.finnjetweb.de/en/
Btw.. travelling "only" to Europe, I do not understand, what is the problem. Europe.. NOT EU, that covers many countries from North of Sweden and Finland, to the UK, Poland, down to Greece and all these countries have damn many interesting ferries and alot of them have a good history. It does not matter, how many countries you travel, it does depend what you do, when you travel. Some people travel many countries but they do it in a superficial way, some travel not so much, but do it in a deeper way, in staying long time and make friends with local people and actually try to experience and not just at a glance.. of course, some ferries have always to be included
I am also not sure about modern interior on the TOR-Line sisters which ran for DFDS. I know the ships as Prince and Princess of Scandinavia, nice ones but after known the Finnjet, not too innovative I think!?