Instead of buying a box packed AC, get separate components of good quality. Like you can buy compressor of Success / Sanden / Denso (all are almost equal if cooling is compared), and buy a good quality Multi-flow condensor, and a good evaporator that can fit in your car. + a good quality TXV and filter-drier.
Rest of the things are piping etc.
Most importatnt is the good mechanic (as said by nawan885)
An other very important thing which our mechanics don't consider is the type of the gas (R-12 or 134). Always make sure that you buy all products compatible to either R-12 or R-134, never mix both. Our mechanics will say that "koi faraq nahi parr ta" but don't use R-12 type compressor with R-134 type condensor and evaporator.
Better you go for R-134 equipment.
Keep in mind that Sanden 505, 507 series compressors are for R-12, don't use them with R-134.
While buying compressor make shure that it is for R-134.