Alot of first time buyers show through the peachparts threads asking about advice on a purchase .This might be a good start in what to look for on any w123 adds showing FOR SALE.
I will start at what is the common denominator ,200k miles or more
YEARS 1981-1985
$1500 is a good starting point with a sound suspension underneath
300cd Add $500
300td Add $800
Motor and Transmission run smooth/ work perfectly $500
No rust add $300
Service records complete add $500
New tires and brakes add $300.
A/C WORKS add $300
New Front suspention rebuilt add $400

Exterior and Interior graded 1 to 10 is priced at your discretion.ADD HERE Its a factor that really is apperance driven but rust being part mechanical WOULD play a part to the soundness of the suspention , a factor .Some people wont touch a rust issue,period.

The basic 300d will give you long and loyal service as an owner ,you have some condition issues with these cars as is the case with every used car ,Its a Buyer Beware World ,or be aware to split hairs.

If you have a range of $2500-$5000 you should find a very nice Mercedes w123 model thats been cared over for those beans .Happy Motoring.
If all the above is a BIG check mark with low mileage ,a 5k starting price would be my new bench mark ,starting there then up .Condition an 8 or better on interior and exterior being the modifier .