Hi Peter
Having imported Japanese cars in the past, I know that Japanese Domestic Market models vary wildly in spec versus their UK equivalents. With people carriers (Previa etc) the JDM versions have smaller/less powerful engines and performance cars like Supras etc often have smaller turbos and weedier brakes. On the flip side, JDM vehicles often have more toys than the UK ones as the Japanese like their gadgets!
Differences aside, I wouldn't expect any inherent issues with reliability/longevity, any more than you would with a vehicle purchased over here, just that its not always as easy to find out how well a vehicle has been treated previously when you're importing it. By virtue of the Japanese laws (and the reason that so many JDM vehicles end up over here), imports typically have a very modest mileage and I wouldn't be as concerned about vans and people carriers as I perhaps would about performance cars which are possibly more conducive to abuse!
It may be worth speaking to David Elliot at Wellhouse Leisure, he does this kind of thing all the time and really knows his stuff; details can be found here:
www.motorhoming.com/directory?sobi2Task=...=18&sobi2Id=7075
I hope that's been helpful in some way!
All the best,
Mark.