The DR500/200 is an interesting aircraft, I don't think that the panel and ventilation was any differedt from the DR400's built at the same time.
The question with this aircraft is one of trading the extra comfort of the slightly wider cabin with the cost of ownership. The Lycoming IO360 overhaul cost is about 30-35% higher than that of the O360 in the DR400/180 and then you have the VP prop that will require overhaul on a four year cycle.
The added performance of the DR500 is mainly in the take off run and climb not the cruise, the DR400 airframe is the limiting factor when it comes to cruise performance with the DR400/160 being the most economical true four seat aircraft in the Robin range.
For those of you tempted by a cheap deal on a shabby thirty year old metal aircraft take a look at this:-
engineering