well i have ridden both passo and alto lapin (the retro style alto). passo is great, has 1L engine, big spacious body. partial digital meter, air bags, full abs, crash protection, perfect AC and good sound system. the car is a treat to drive. just buy it and drive it around 2.5k rpm and you will get best mileage. parts of this car are available but on request; dealers import parts under the seats, in the boot or other hidden spaces in car to save duties.
alto lapin is amazing and fun car, immaculate interior, every thing powered, comfortable seats, abs, hids and 660cc engine. its a treat and also cheaply available. auto gear with steering so you have space for three in front row. perfect ac cooling even in full sun at around 1pm standing in traffic lights.
as compared to mehran that car is miles ahead. forget parts, you will get em as car gets popular dealers arrange for parts like vitz; go to market and you can find any part of the vitz you want.