The single most useful thing to understand before buying is that an air purifier cleans a room, not a home. Purified air does not travel meaningfully through doorways against normal air movement, so one unit in the hall does very little for the bedroom where you spend eight hours a night.
That reframes the budget question productively. For most Indian families, ₹30,000 is better spent as two ₹15,000 units in the two bedrooms than as one ₹30,000 unit in the living room. Prioritise by hours occupied with the door closed, and the ranking is almost always bedrooms first.
The second thing to get right is size. Manufacturer coverage figures are measured in a sealed room of exactly that size with no ongoing pollution source — conditions your flat will never meet. Take the claimed coverage and treat it as a ceiling, then buy one class up. In Delhi NCR or Ghaziabad in winter, two classes up.
Everything after that is refinement: carbon depth if you are near traffic or cook heavily, sleep-mode noise if it is going in a bedroom, and above all the cost and availability of replacement filters — the purifier that stays in use for ten years is the one whose filters you can still buy in year four.