1. Do a search on cruise control. This topic has been discussed many times.
2. I don't think the LCD is repaceable. I'm not sure what you can do about that?
3. If the SRS light stays on for a minute or so after you turn the car on, then there is no resistance in the seatbelt buckle of your seat. This means you need a new buckle (about $75), it's fairily easy to install yourself, but you need Mercedes to turn off the SRS light.
4. If you look at the cassette holder, and look up above it, but below the radio, you'll seee 2 screws. Kind of hard to explain, the controls are all vertical, but there is a place below the radio that the dash is horizontal, look below that, and remove the 2 screws. You should be able to kind of force it out from there. If you can't, then you need to remove the ashtray and the pocket below the ashtray, but if you try without doing that, you should be able to get it ok. You can buy a storage pocket that went into the later C-classes ($11), and then put you IR reciever in there... will probably look nice and more complete.
Lastly, congratulations on your new purchase!! And welcome to the forum.