Congratulations on choosing a good solidly built van.Providing it has been serviced regularly the engine is designed to go around the clock.Fiat did make the 2.8 TDI from about 98-01 I believe.It's a direct injection 120 hp unit which is which is not the fastest but will easily keep up with traffic on the motorway.It may be necessary to change down for the long drawn out hills but not a huge problem,most of the owners I have spoken to speak well of the engine.
There's not much goes wrong on Hymers of that era,they were built to last.My opinion is that the Hymer golden years were between 98-06.Check all the fitted appliances on a vehicle of that age-fridge,hob,water heater,heating,and water pump.The only other things I would look out for is correct operation of window catches and skylights,rust on the front chassis member,operation of the drop down bed struts and fading of the front cowling and rear capping but you would have doubtless have checked that when you bought it.