Pulling the engine to change the water pump?  Who told you that?  I'll
be nice and simply say to disregard anything else that person says.

To get to the pump means the timing belt has to come off.  Typically
probably around $300 for the labor.  The water pump wouldn't add much to
the labor cost.  If you say what city and state you live in, there might
be an experienced owner around there who would do the work for a decent
price, and probably do it much better than most places, including
Mitsubishi dealers.  Or do a search on Club DSM and join your state or
local DSM list and see if anyone wants to help.  Oftentimes people will
help for free if you provide the beer.

My concern is your driving that last mile "slowly" with the temp gauge
in the red.  If anything, you should have driven as fast as you could to
get the car shut off.  If it wasn't pegged fully hot, then there might
not be any serious damage.  A compression test should show any serious
problems, and you don't need a good water pump to do a compression
test.

here is everything you need to know about the subject:
www.vfaq.com
you do not have to completely pull the motor but you will have to unbolt the
motor mount so the engine can be raised and lowered. this allows access from
up top and through the fender well.
you will want to replace the timing belt, balance shaft belt and the
tensioner for the timing belt.
you can find parts here( go to the club dsm section under mitsubishi)
www.partznet.com