I rowed and then coached crew for 5 years. As noted, Concept II is the erg to get. But, like anything athletic, you need good coaching on how to first use the machine correctly and then get the most out of it. It would be money well spent to have a crew coach or someone who knows how to row teach you how to use an erg properly.
If you are looking for a workout, get the model B. For whatever reason, it provides a better workout than the newer models. I suspect the newer ones had design creep to make them easier for general consumer whereas the model B was made only for rowers in mind. good luck!
And be careful what you wish for. Ergs can be brutal when used properly.
Never leave the handle on the catch when not in use like in the pic below
