What legal points should I review in the SPA?

Do not treat the SPA as a formality.
It is the document that governs your relationship with the developer or seller when everything goes well—and when it does not.
Focus on the clauses that materially affect your decision: is the handover timeline clear or vague, what happens if you default or the developer delays, are there administrative charges on assignment or resale, and how are size variations, defects, or handover procedures handled? The clearer the SPA is, the safer and more predictable your decision becomes.