Stereo & Trip broken?

Both the stereo and trip computer are playing up in my 93 850ci
The stereo can be a pig to turn on and then randomly turns itself off
I assume its a problem with the main on/off button?
Also the "check" button on the trip computer has just stopped working
(I may have broke it by pressing it a little hard?)
Are these units serviceable or are they compatible with other BMW's ?
Good used might be easier to find?