Regular users on here are all enthusiasts, Hence most will recommend going straight to Standard or Pro. Don't let that put you off dipping your toes if say you are only beginning, or just want some nice sounds.
There are simple and more complex ways to modify the sounds in Pianoteq. Someone who just wants a little bit of sound adjustability and plenty of instruments will be happy with Stage.
However if you are fairly comfortable with using computer software (for instance already make music in a DAW), you like the idea of being able to heavily modify your sounds, combine instruments to play together, move virtual microphone positions, and change microphone types, and the number of them.
Lift the lid, blend instrument models together, load other users free presets from the forum and so on you will eventually end up with Standard or even Pro.
Plenty of folks will be more than happy with Stage, however Standard recently became a substantial upgrade over Stage with the edition of Morphing.
That basically allows you to quickly and easily blend the various physical models to make new original instruments without having to understand pretty much anything!