You lucky man Fritz, these are a special car.
a) Hirschmann radio's can normally be repaired. Google them and see if there is a dealer in your area that could order parts for you. Otherwise, check out Woodyscustomshop on ebay and you can buy new ones.
b) Get a Hemmings and look in the Mercedes section. You should be able to find used ones. Otherwise, try Tom Hanson at the Classic Centre in Irvine CA for new ones. be careful - the heater taps often sieze - don't try to force them with the handles.
I have bought coupe parts from Jon Brooks before, and he shipped them to Australia very professionally - firstname.lastname@example.org
c) Gee, don't know about 280 but on 250 and 220 cars the temp gauge is a capiallary tube, which once broken, is stuffed. If it is a capillary tube type, it will need to go to a gauge repairer. Budget on perhaps $100-120. Please don't go the cheap crappy solution of putting another gauge in - these cars are special and deserve a little bit of money to be spent on them.
d) Speedo could just be a broken inner cable. Again, when talking to a gauge repairer about c), price up a new cable. In Oz, it may cost about $100 again. undo the cable from the rear of the speedo (you will have to get contorted up behind the dash, and at the trans, and get someone to spin the inner part on one end and see if the other end turns. If the speedo iteself is stuffed, a gauge repairer should be able to get it working again. Hemmings has lots of gauge repairers listed. If you want MPH speedo, Jon may be able to help out. As I live in Oz where metric rules, I would be interested in swapping MPH for KPH if you are not worried about originality.
e) The Classic Centre has workshop CD's you can get. You can PM me too if you want me to scan or email certain bits.
Drew in Oz