I have an excellent AC in my Suzuki FX 1984. Its very nice even in Scorching heat of May and even at the Noon but only for front two persons at noon and for every one in the evening. The engine is not in a very good condition so fan in direct but even in heat the needle does not cross the 45 % of the heat mark in rush other wise on empty roads its around 30% on CNG and around 20 % on petrol. The car has the original AC but I have changed the compressor to the Japanese 660cc Mini Pajero Compressor which does not add a lot of stress to the engine and has an excellent cooling. You need to change the pulley as the original pulley of mini pajero is grooved type.
I have a 2005 cuore with a factory fitted AC. i got it full serviced. performance is ordinary as compared to other cars. it is chilling in morning and evening. but when car is standing in sun at noon for hours its AC works just OK. many times i turned off AC and opened windows but air outside was hot as hell. so i say its a good idea to go for it. and try to park car in shade. as for load on engine, drive on 35-45 in second or 60-70 in third gear. at these speeds AC works best in small cars with almost no load on engine.
Boss i had mehran for 4 year and i installed denso ac (Ac recommended for all honda, toyota, etc) and never had complain even on signals, just install denso, the AC wala's will force you to install sandan but if you want good chilled blow i will suggest/request you to please install denso its bit expensive then the sandan but you will never regret it.
Moreover suzuki is also installing that in new Mehran.