the furthest I have gotten so far is Budapest.

so this upcoming trip I am looking at heading into Rumania, Sofia, Belgrade, Greek coast, Croatia,

how is the best way to get around?

I am looking at taking Eurostar from London to Cologne, then rent a car for Berlin/Prague/Warsaw/Budapest

trains seem like a real pain in a hard language with not everyone speaking english. that is why I thought boat my be better. or car rental but that seems like unless it is with SIXT you are driving some Lata with Igor.

I typically rent a car. I have way more flexibility to do and see what I want while avoiding wasted time waiting in train stations. And more often than not, the regions & small towns between cities can be just as (if not more) fascinating than the cities. Those "off the beaten path" places are where the real culture is most of the time. The only problem with driving is navigating the cities themselves.

I can't speak to Romania, Bulgaria, or Serbia, but driving in Germany, Czech Republic, Hungary, Greece, and Croatia are all easy. I will be driving all over Poland this summer, and from what I've gathered, it is easy as well.

I usually use a car rental site like rentcarla.com to compare prices & book, but there are other similar options as well.

How long is your trip? If between 21 days and 6 months you can do a long term auto lease (e.g. Renault Eurodrive or Peugeot Open Europe). We rented a Renault and it worked out really well. Put around 15,000km on it over six months... drove from Paris to Scotland, across Scandinavia and from Estonia to Greece. Driving down the Adriatic coast from the Istrian Peninsula to Kotor was a definite highlight of the trip.

FYI, when I was in Germany a few months ago, I didn't find a single parking garage that took credit cards. Euros only.