Hi Albert,
I've never done the evap or heater core. I think you have to pull the heater core out to access the evap - not fun. Pulling the dash is not too bad, just tedious. THe procedure is in the factory manual, it's pretty basic. You will have a LOT of little parts, so have a workbench are or some place you can set each item & the screws for it separately. And it should be somewhere that it can stay apart for several days if required! Definitely replace ALL the vac pods while the dash is out, particularly the center vent pod. Photo of the heater box is here:
http://www.meimann.com/images/mercedes/W124_dash/top_removed.jpg
More photos:
http://www.meimann.com/images/mercedes/W124_dash/
Note that the pipes leading into the evap are also hard to get to, at the expansion valve, under the windshield trim , behind the brake booster. It's definitely a major procedure. I think you could do the dash removal yourself, if you follow the factory procedure carefully. Acutally pulling the core & evap is what I don't know about - I'd ask your friend and see what he says! Just curious, are you *sure* both evaps are bad? Maybe it's just a failed O-ring at the expansion valve? I don't often hear of evap failures on the 124 chassis...
Regards,