The W123 is generally known as one of the best cars that M.B. or any other manufacturer ever produced. A properly sorted out and maintained W123 will outlast any modern model.
In Africa and the middle east the W123 and W124 are still the taxi of choice particularly the diesel engined models.
W123 230 engines were designed for leaded fuels and no Kats. They run optimally on premium unleaded fuel. They will not give the economy of a W201 or any later contemporary vehicle. Depending on the year the car will either come with K or KE jet injection - most are the simpler K jet. M102 engines are overhead camshaft and don't require valve adjustments. (A M102 engine is a M103 engine missing two cylinders)
You have access to the internet so you will find that all parts are available for these models.
In Australia there are still thousands of W123's being used as daily drivers.