I took this tour in 2012. Our guide, Peter Polczman, handed out a detailed itinerary on the first day of the trip, including suggestions about amount of currency needed and reminding us along the way to either get (or spend). It does depend on how much eating out/shopping you plan to do. Some of us pooled coins when we were on our way out of a country to buy snacks/treats at rest stops. I also gave my guide the remainder of my coins at the end of my tour, since he would have need for them again. I printed out an OANDA cheat sheet (https://www.oanda.com/currency/converter/) for each currency and kept the sheets with the money separate in small plastic ziplocks (kept in my money belt). At that time, I got some currency ahead, but I would just do ATMs these days. For reference, you'll be dealing with five currencies: Czech koruna (CZK), Polish zloty (PLN), Euro (Slovenia, Slovakia), Hungarian forint (HUF), Croatian kuna (HRK).
You only need the usual euro adapter plug (like this one).
No specific suggestions for this trip in terms of gadgets, etc. But I will recommend bringing home as much Hungarian paprika as you can carry (it's very inexpensive at the supermarket) and also add at least one day in Prague ahead of the tour. I had a really fun time in Krakow doing a Crazy Guides trabant tour (on my own time)