I did my daughters '90 190E 2.6 about a year ago. It blew out on her and burned both of her ankles. To make sure it is the heater core, disconnect both of the hoses where they connect to the tubes coming through the firewall (those tubes connect directly to the core itself). Then plug one tube and connect a source of air pressure to the other one. If it dosen't hold pressure, its either the core, a tube or the connection of a tube to the core. I wouldn't use too much pressure, I used a tire pump.
The air bag is held on to the wheel with screws from the back. Seems like they may be Torx head screws. I couldn't imagine taking the dash out with the wheel inplace. The rest of it is all by the numbers, just follow the Haynes 2.3 manual. There may be one ot two things different between the two cars but not enough throw you. I didn't get in any hurry and I think it took me one day to take it all apart and another to get it all back together. I don't remember any one thing that was too difficult. I'd just love to know how they design all that stuff to fit together like it does! Amazing! Give a whistle if I can help in any way.
p.s. I did the engine on my '60 356 Roadster