If the compressor is a reputable (Toshiba, Gree, Carrier, Mitsubishi) make you can buy an e-cheapo branded unit, non-inverter type. The only problem is the cheap aircons generally have an ugly indoor unit (midwall-split type). I've personally owned both Infinity and Jet-Air branded aircons (Toshiba compressors), both bought from a installer who offered 4-year warranties on the compressors provided I have them cleaned/services annually for R250 a service.
As for the best prices - I found that calling refrigeration installers as you see their bakkies generally results in good deals. Buying from retailers like Game typically does not include installation and you end up paying R5000 for a 12000btu aircon + R1500-R2000 back-to-back installation from the installer, whereas if you buy the aircon from the installer, you can get away paying R4500 (or less) with installation.
Game has a 12000btu Defy midwall split with installation for R5000 on promo this week. 2 year guarantee.
Makro has a Samsung 12000btu for R4400 (promo) excl. installation or Goldair 12000btu for R4000 excl. installation.