Well you have already narrowed down your options quite a bit. It seams like you have already decided. What advice do you need?
Some late model TownAce vans, KR42, have a 7K carby motor in them. This would be the sort of thing you want. The very newest TownAce vans, also KR42, use a EFI 7K-E. Some older vans used a 5K carby motor. This is also something you may want.
If you go with a 4K, you will need to work it to get any decent power. Start thinking about a different cam, different carby, and different exhaust as the bare minimum. You also want new valve springs and things like that. 4K's are very cheap, so fun to play with in that regard.