I bought a "水貨車" from Kowloon Bay several years ago. It looked OK and the garage also offered it to be checked by AA just to give me extra peace of mind... They didn't find anything wrong, so I bought it. Problems came afterwards...
1. insurance will always be higher than "行車", sometimes a lot higher or even not insured, depending on the model
2. maintainance will also cast a bit more
3. the car may have been "flooded" - which is what happened to mine...
4. milage may have been clocked....
obviously there are loads more potential pitfalls.
I am not saying that all "水貨車" are not trustworthy, you just need to be a lot more careful if you do decide to buy one that's all.... good luck