Regarding the heater core .... it would be a bummer to have to go back into the dash to do it later. More parts would only include the heater core, O-rings which should come with the core.
Labor .... well it does take more effort since the heater core is inside of the control unit box (for lack of better description). How many more hours I am uncertain as to the amount. My guess that it would add maybe another 3 hours +/- . . ....
The nice thing about doing it all at once is you know (peace of mind) that all those buried parts are taken care of, sans the heater motor and evaporator core that are accessed anyway from the otherside of the firewall.
Are you planning to make it a DIY'er?
Keep us posted,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)