The Merc 208d will only be 80 Horse Power so will not be the fastest thing on the road, in fact it will be quite pedestrian and barely able to keep up with modern traffic on fast roads. Our 2004 Merc Autotrail is 130 HP and still is on the slow side of things.
As for the Autotrail side of things, everything depends on how it has been cared for over the years. If it has had water ingress and damp then it may or may not have been repaired properly so make sure you get it inspected by someone who knows what they are looking at or be prepared for plenty of DIY. Also the appliances fitted, if still original, will almost certainly be obsolete so be prepared for problems if they go wrong.
Keith.