I had the same thing happen to me on my 190E as well. I bypassed my heater core for about a year or so and finally replaced it this past december. It wasn't as bad as I thought. Here is a quick list of things that you will have access to that you may want to replace (all are optional):

1) two vacuum pods
2) brittle, plastic vacuum lines + rubber connectors
3) replace/repair switchover valve (mine was britle where the connectors were and 2 of them broke off so I replaced it)
4) instrument cluster lights (and while you have the cluster out, if you want to do anything with it, now would be the time, i.e., replace fuse, change dial, repaint needles, etc.)
5) clean up vents/ducts
6) replace evaporator
7) replace lights inside ash tray and climate control
8) replace/repair blower speed switch (if you have the one that slides, there is a plastic piece that breaks inside the unit that will prevent the switch from sliding to the desired speed)
9) check ignition (sometimes the 190s key won't turn in the ignition, not sure what causes it)
10) speaker/radio upgrade
11) throoughly clean the dash while you have it off with some cleaner and condition it with some Lexol leather conditioner.
12) Also, may as well flush your cooling system too since you'll need to drain it again.

Again all of these things are optional, but hey, if you have the time and are able to get some of the replacement parts, you might as well go for it.

As na side note, when my heater core exploded on me I'm lucky I wasn't driving, I barely turned on my car and it gushed out all over my leg and the driver side floor....because I like to make sure things are extra secure/protected, what I did to my new heater when I got it was put an extra seal of clear water epoxy that can withstand 1500 psi around the edges where the aluminum meets the plastic and I also put a coat around the plastic part where you connect the aluminum pipes. I wanted to put a sleeve over the aluminum pipe all the way up to the plastic part so if it did leak again he the coolant would drain out of the car.....a sleeve would not fit though:-( just my 2 cents. Good luck.

__________________

1990 190E 3.0L