Ive just got back recently from the European Pioneer tour (36days), and maybe able to help.
I booked through a travel agent and shopped around, first speaking to the top deck representative at the flight centre travel show, and got some info from them, they are always helpful. I also spoke with STA travel, who matched the same deal that flight centre were offering, and ended up booking through a friend who does corporate travel, who also matched the deal. I think that they basically all offer the same deal, and if they know someones doing it cheaper, they will match the deal. Payment methods that i was told by all the agents were a $200 dep, then pay the balance, by a particular date closer to the departure date of the tour. To get the discount that i got, i had to pay the balance 6 months before departure.
As Morgan said Guarantee departure, diffidently means they will leave, no matter how many people they have booked on the tour.
Regarding the money thing, obviously everyone is so different. For me i allowed 100Euro per day, and there was no way i spent that much, but i did not buy alot of souvenirs. The biggest spend was on food, but i ate very well, and in good, cafes and restaurants, you can go to stalls, markets etc… where it will be alot cheaper. I think in the end i probably spent around 2000Euro, including the optional’s that top deck offered ( I did most of the optionals that the tour offered, but the most expensive is skydiving in Switzerland, that i did not do), and entrance fees, and public transport etc… that were not included.
I hope this helps, let me know if you have any other questions…
Enjoy your trips, its well worth it!!!