I've done this one. The new "water pump" in your hand is the insert which bolts into the water pump's housing. The only thing that's really new is its bearing and seal. So that you'll appreciate your mechanic's offer I'll describe the job.
You'll need some metric sockets, a short 4" or 6" extension, ratchet and open end and closed end wrenches. If you lack these and feel that they are "instruments of the devil" have the mechanic do the job. QED
The water pump is located on the front of the engine, behind the belt driven fan. The coolant must be drained into a pan; the drain plug is on the bottom of the radiator, on the left. You get to it by lying under the front bumper. If the coolant hasn't been replaced in years it should be.
The drive belt's tension needs to be slackened; that's done at the alternator. Its mounting bolts need to be loosened (only) so the adjustment screw can move inward to relax the belt. You'll be plenty dirty after this.
Remove the top radiator hose from the thermostat (water pump end) (only) and gently stuff it somewhere out of the way. Now comes the tough part.
There are four or five small bolts on the front of the belt driven fan hub that need to be removed. The work space between the radiator core and bolts is tight and the bolts are tight. Some people remove the radiator. IT'S VERY EASY TO ROUND THE HEADS so be absolutely sure your closed end wrench is the exact size and fits perfectly. If you round one of them the job is lost to the mechanic after a tow to his shop. Remove the fan and hub; stand the fan vertically - don't lay it down. Now it's time for the water pump.
Your car's engine has a large harmonic vibration damper with a cut out portion that you need, so rotate the engine with the key or your hands so its cut out allows your socket access to the water pump's lower bolts. Use a socket to remove the five or six bolts that secure the rotating core from its housing. Remove the core and scrape the gasket surface of the housing clean so the new gasket will seat and not leak. Install the "new" pump with the gasket in place; the pump mounts only one way or the bolts won't fit so that's easy. Torque the bolts to about
20 ft lbs.
Reinstall the fan hub and fan, tighten the new hub bolts with a closed end wrench as tight as your hands can stand. Reinstall the drive belt and tension it at the alternator, secure the alternator's mounting bolts. Reattach the upper radiator hose to the thermostat end. Replace the radiator's drain plug and refill the coolant.
This is the job. It's not mentally difficult but has a few nasty bits. IMHO your mechanic is earning his $270 for it requires tools and takes at least two, if not three hours to complete. And remember, he has to deal with any rounded bolts etc. I found the bent over position of the job gave me a sore back for the next two days.
Happy New Year