What happens when you change the timing belt and head on a 95 Villager and it still locks up?
Villager engines are an interference engine.Simply put it means that if the belt breaks when the engine is running or if one changes the belt and installs it improperly then attempts to start it the pistons will come in contact with the valves.The next time a start is attempted the engine will not turn over. Another possibility may be that you are getting coolant into the cylinders from improper installation of the head gasket.Liquid in a cylinder can cause a condition called hydrostatic lock.As the piston cannot compress the liquid the motor will cease to turn over.
That is not EXACTLY accurate...when it was manufactured as a Nissan quest, that was true, it was an interference engine. When Ford motors contracted for it and it became a Mercury product, they demanded that it be a non-interference engine. I have lost the timing belt tensioner while driving and the head was fine. I would look for problems with the head installation itself as opposed to the timing belt having damaged the valves. Also, make sure that the belt was installed correctly, that the plugs are firing correctly, etc...
Villager engines are NOT interference engines, this is a myth. If you have installed a timing belt and the engine won't turn, either your timing is far off, or you had a serious mechanical fault before.