The most important thing is for them to lots of research before even thinking of buying. There are all sorts of layouts and configurations out there and while some of them might look ideal your parents need to consider whether the layout will *really* suit them over time.
The best way to do this could be to spend a weekend nosing around other peoples campervans in a campground - most people don't mind the occasional browser and I know a number of people have approached me to have a quick look around mine for ideas.
There is a pretty large number of sites out there containing 2nd hand campervans complete with layouts, specs, etc . . . so a long browse of the internet wouldn't go amiss. There are almost no Irish dealers, so don't go looking for specifically Irish sites - UK & German sites will give the broadest range.
Also check out the various Campervan magazines, but bear in mind that they will be reviewing the latest and greatest and unless your parents are going specifically for a compact 2 berth there won't be anything like that in their price range.
Most 3/4 berth new ones are in the £20-30K price range, so that's about €30-40K.
Also, they need to bear in mind the difference between UK & Euro spec campervans - most UK models seem to include an oven even though I cannot see anyone doing baking or roasting in a campervan. Euro models come with left hand drive (or is it right hand drive ?) and a door that opens onto the roadside rather than the footpath. We have a Euro one but don't find this to be a problem.
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