If your serious about doing it and can afford it then definitely a Concept 2, theres a very good reason why you see them and not other companies products in most gyms and leisure centres.
I looked at all the options and bought the Concept 2 Model E for just over £1K, you can get the Model D for£850 new, the difference is the monitor and interface with smartphone apps, I’m geeky about things like that and like being able to use all the smartphone apps.
You can pick up used/reconditioned ones, I would be wary unless they have some degree of warranty just because you don’t know how worn they are, or whether they have been looked after.
I bought mine in Jan and got the bug big style, I was rowing 10K’s a day four five times a week, unfortunately a scorre related injury then stopped me in my tracks. I’ve also developed tennis elbow, all down to me getting stuck in without sufficient stretching and warm up, I’m now off rowing until I can sort my tennis elbow. What I’m trying to say is don’t just buy one get it and row either get some coaching to be sure your technique is right or use some of the internet resources available – Dark Horse Rowing are brilliant.