My four year old car has died...

The timing belt gave in and with it a couple valves as well.
The timing belt didn't break, it had some threads that fell off, and
when the car broke down I was actually slowing down to a stop so I
could back up into my driveway.

I don't know much about cars but I was presented with three choices:

-Have the engine open and see if it is repairable. Cost : 4 hours to
open it up ($50/hour) + around $1100 for the repair (if it's
repairable).

- Replace the engine block with another one that has 122000km and also
four years old for $1600 installed.

- Or replace with another engine that has only $48000km and also four
years old for $2000.

This would be a no brainer but it seems that only the top part is
replaced, the one where the pistons and valves are.
Which means the other parts of the engine remain and those still have
four years and 200000kms in it.

What to choose?
I've been told that I should go for the 122k part because not only I
save the money but because the the rest of the car will start to give
in before the block and by then it will be time to get a new car
anyway.
I have also been told to put the 48k block in it because if I plan to
keep the car until it falls apart then it's better to use a less used
part.

I drive 48-50000km/year so I don't expect to keep the car forever.
Does it make sense to put a 48Km block into a 200000km car?

Thanks for all your comments.