w123 300d with a manual transmission was the best car in the series. is yours a four speed or the 5?
regarding the paint, its no big deal. any good paint dealer will have the shade card of mercs and so you can find the very shade you want.
w123s are known for their horrible vacuum leaks and the headache they give, fortunately you've located the problem, which happens to be the tough part. its easy to fix if you know what the problem is.
wheel caps do not worry, there are plenty of w123 parts available with scrap dealers, but you'll have to paint the middle part.
the key falling off the ignition is quite funny, again scrap dealer to the rescue.
regarding your non working odo, its a DIY.
remove the driver lower panel of the dash, and reach up the dash and slowly push the instrument cluster out.
once its out disconnect the socket and clock wires. the oil pressure pipe also has to come off use a 10mm wrench to unscrew.
once you have the cluster out unscrew the and remove the clock/tacho and then the speedo. be careful not to damage the trip reset lever while you are at it.
the problem could actually be that the plastic worm and gear inside the speedometer have moved away. all you have to do is to push the small rod holding the whole assembly from the other side towards the worm and you are sorted.
once this is done you can lubricate the gears with some wd40 and refit the speedometer to the cluster.
and then reverse the removal process and you have a working odometer.
How to Repair Your Broken Odometer would give you more details. i followed their instructions and it went well.