Hi Kevin. The 190D badge has been used a few times in Mercedes-Benz history, so I assume you are referring to the 1980s model code-named W201?
These models certainly have a reputation for being very well built and engineered, but being anywhere from 25 to 35+ years old now, prices will vary enormously according to condition and history.
I’d get as much info as possible from Mercedes owners forums and magazines, which should give you an idea of what to look for. Monitor the market for a few months at least to get an idea of what good cars are selling for, and have a look at a few different vehicles before taking the plunge and buying one.