You'll get the additives out when the motor comes out, there will be almost nothing left in the system and as long as you use new coolant you're fine. Yes the motor is easy to pull in the FM platform cars, $2100 is a pretty darned good price for having someone else do the work. Something you should consider replacing or at least THOROUGHLY inspecting before installation is the motor mounts, the OEM ones start showing significant wear by 150k (moreso on a 6MT but a 5AT equipped car can still have a lot of wear on motor mounts it just depends on how it was driven by previous owners) and since you're over 100k and will have the motor out it's something that I personally would replace. OEM rubber definitely gives the best overall quality but if you aren't concerned with NVH and want a performance upgrade (not sure if this is a concern since it's for your son anyways) the urethane motor mount upgrades make things very rigid.

 

No you buy the homeless guy the biggest, greasiest, spiciest, most deep-fried food in town and then have him relieve himself in the driver's seat of the Shady Salesman's car, make sure it goes all the way to the floorboards.