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.

The older M102's do not have valve tappets and do need valve adjustment. It wasn't until 1985 and W124 that they converted the M102. But the older M102 is probably one of the best gasoline engine ever produced with some seeing well over 300,000 miles on the clock. I own a 1983 230E that was missing it's dials when i got it but i suspect it has around 200,000 miles on it and there is no letting up.

__________________
600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.