I'm using RX-Water Filter, but when I try to buy a new cartridge, it cost :

1) Cartridge 1 and 2 (1-2 year) - Rm 298
2) Cartridge 3 (2 year) - Rm 275
3) Cartridge 4 (3 years) - Rm 4++ something

Damn expensive .. so, consider the cartridge replacement too in the future ..

Yes, the smaller the better, depends on what is the end results you aim to achieve.

you wouldnt want ultra microns, although it removes most contaminants in the water, but its also remove all beneficial minerals (like calcium, magnesium, phosphorus) in the water. not advisable for long term drinking purpose.

You want a small microns so that can remove most harmful contaminants in the water, including pharmaceutical drugs, organic and inoganic contaminants, herbicides, pesticides, heavy metals etc.

Some filter that have sizes less than 0.01 microns claim to remove 99.9% of bacteria in the water also, but to be safe, its still advisable to boil the water after filter (unless the filter comes with UV to kill of water borne bacteria and viruses)