Your first guzheng does not have to be your last, but stable tuning, a comfortable action and the right size are worth insisting on — they shape both how practice feels and how quickly you improve.
Stable tuning comes first. An instrument with uneven string tension or slipping tuning pins will spend every session going out of tune, which slows the development of your ear. Ask a teacher or an experienced player to try it, and check whether it holds pitch after being strung up.
On tone: a brighter, more open instrument suits contemporary repertoire, while a fuller voice with longer sustain is more forgiving on traditional pieces. Neither is better — it depends on the repertoire you are heading toward.
On size: the standard instrument is about 1.63 m and the compact one about 1.25 m. The standard size has the full range and fuller resonance, and suits systematic study. The compact instrument is easier to carry and store and works where space is limited, at some cost in tone and volume.
Action deserves attention too. Tight strings are hard work to press; very slack ones are hard to control dynamically. Try several instruments with your teacher and find the one where pressing does not feel like a struggle.