Buy what is affordable - all of the above will turn a pen barrel. In general, the less costly ones have less HP, less features, multi-functionality (use it to drill)
possibly less weight and length and accuracy. However, other than it being less accurate or "true" (center points meeting, tail stock holding, etc.) , it will turn a pen and you will have just s much chance of enjoying the finished product. In the long run - after a few dozen pens - you may be likely to learn that the increment cost of a more expensive lathe is quite small when you consider all the other costs.
If you buy an inexpensive one, you can always use it as a buffing station, pen press, finishing station
if it doesn't work out. There seems to be always something that you can do with something that spins

. I turned pens on the smallest HF (they don't even make it any more); I think it was $80 and 1 morse taper. I just didn't ask it to be something it is not. I bought it to as a CA finishing station, but wound up turning a pen on it. In my case. it was fine. Remember, even the smaller Jets, Rikons, etc are "toys" to a certain degree when compared to the Robust, Oneway etc. The lathe depends on what one asks it to do. I own 3 Jets and 2 Rikons...I call them "toys" not to disparage them, but to contrast them to 3 to 5k lathes.
Enjoy the journey!