I've never played the YDP-121 before tonight as I always thought of it as a low end model but I tried it out tonight as I remembered your thread.
In my opinion, it's perfectly suited for your first year of piano lessons.
On the other hand, although some piano salespersons attempt to rip you off, I think I understand why he's suggesting you purchase a higher end model. The YDP 121 may be suited for one year of lessons but what about after that? Are you going to purchase a better model in the future? If you can afford it, than don't let it bother you. Do what YOU think is best and DO NOT under any circumstance let a salesperson talk you into purchasing something you don't want. Play all of the models in your price range and decide which one you think is best. If you only need the YDP-121 for one year, can afford it and feel it's a worthwhile investment, by all means, get it.
Compared to some of the digitals I played tonight (Suzuki, Korg, Roland, etc.) all of which were very low end models, the YDP-121 sounded pretty good, all things considered.
After that, I moved onto the Yamaha Grands.
Needless to say, you're lucky I have such good memory or my opinion of the particular piano in question might have been hindered.