Completely personal opinion:
The only way to determine "under" powered is if it won't pull the intended load. If you're not going to pull a trailer or overload the cargo area, then most available engines SHOULD be powerful enough. After all, it's a VAN, not a racecar.
More than just an opinion, its advice. Have had a T2 type 25 and T4 and wouldn't advise buying a T2 unless your prepared to put money into it on a regular basis. You get lots of these in the US where the climate is kinder to them. For the cost of buying a good T2 you can buy a more reliable newer model that shouldn't need to be repaired as often.
I found I ended up using mine as a daily driver instead of a car due to the fuel economy being pretty good.