If you are considering one of the Asian sopranos from ebay, not worth the trouble. They are not all that bad from a mechanical point but all the tweaking in the world will not fix the intonation issues.
The question regarding this thread is: "what is your definition of 'student horn' ?
If it is an asian cheapie....you know the ones you see for $250-600 brand new.......indeed, forget it.
But, you see, the following contemporary horns are considered by many, whether erroneously or not, to be 'student horns':
~ Vito-branded Yama or Yama 21, 23, 280, etc
~ Modern Evette-Buffets and Buffet 100's
If you wanna throw in vintage horns, nothing wrong with a Conn Director or 24M, King Empire or Cleveland, Martin Indiana, etc...
(once again I am left shaking my head at the 'best you can do is buy a Yamaha' suggestion....yes, it is ONE of several options, but it is also the 'default' option and there are qualities of other established student models which actually exceed the qualities of a Yama student horn. So, no...there is not one 'best' choice, there are quite a few 'best' choices)
But anyway, you get the drift. These noted above (and others, I mention just teh ones I am confident about having seen enough of 'em) are all several rungs up the ladder from the cheapies. And if you buy one of these (quite cheap second-hand, BTW)...then the only 'transforming' you would ever have to do is with the mouthpiece and reed setup (other than keeping it in good service).