I have been a DR400 owner for the best part of twenty-five years and the first thing to say is that the aircraft must be kept in a hangar but just like your Cessna the money you spend on hangarage you will save on maintenance.
You will have to choose your maintenance provider carefully as the lack of understanding within the industry of wooden structures is second only to that of composites.
There have been W&B issues and it has been discovered that at least one aircraft has left the factory with W&B data that was serously in error ( we had it on the weighing kit twice to be sure that we got the numbers correct). Unfortunately I have not tried to do a load sheet for the carrage of four adults so I can't tell you how well it can be loaded.
Have you considered the DR400-180? These can be run on 91UL and are well known as good weight lifters, fron the south of the UK you can get to the north of Spain with one fuel stop and if you have the supplementary tank under the baggage bay you can get to the north of Spain non-stop.
There are no major problems with the airframe but you need to get someone who knows the type from a maintenance point of view to look at anything you intend to buy.
The after the C172 you will find the DR400 a much nicer aircraft to fly with a much greater range of loading options, in theory the EcoFlyer should be a great touring aircraft as 75% power is avalable well above the 8000 ft limit of the Lycoming powered aircraft enabling the aircraft to fly at higher TAS and take advantage of higher tail winds all with a low fuel consumption but you need to sit down with the flight manual & W&B data to see if this all pans out into a practical aircraft for your group.
Perhaps BACKPACKER might like to comment on the loading issues as he flys the aircraft on a regular basis.