Actually there are several threads on external water filter, just need to read them all and you will be able to decide
https://forum.lowyat.net/topic/1869488/all
https://forum.lowyat.net/topic/2340365/all

Just a brief summary:
Despite the many brands in the market there are only a few types of filter
1) Hollow Fibre Membrane 0.01 micron (Eg : LecHome, Nesh, Rubine, Hydro1)
2) Sand Filter 5 - 10 microns (Waterco,Backgree, Perfect Steel)
3) Cloth type membrane 5 - 10 microns (Diamond, 3M)

I do sell all these 3 types of filters but only recommend and display no2 and no3 in my shop, no1 not recommended.

Water Quality
Well, Hollow Fibre Membrane filter with 0.01 micron has definitely the best filtration over the other types but as we discussed earlier on drinking filters like Amway 0.2 micron, 3M 0.2 micron and Aquaphor 0.1 micron, dont you think 0.01micron is just too clean for an outdoor filter? it is even better than the drinking filters Outdoor filters main purpose is to filter off sediments and to prolong the life of your indoor filters and tap fittings.

Imagine our drinking filter pressure is weak, how would 0.01 micron effect your incoming water pressure? It will definitely reduce 1bar of pressure or more.

Other things to consider such as durability, maintenance, maintenance cost and etc you can get the info in the other threads, google on them or drop by my showroom, kinda lengthy to retype everything here.

Generally speaking for landed properties with high water usage i would suggest getting Perfect Steel, i tried 3M AP902 close to 2 years and recently replace it with perfect steel. Why? I have upto 6 people at home with high water usage (10" Sorento Shower head and a strong Grundfos water pump), 3M AP902 capacity is just not enough for us, that is why we opt for Perfect Steel with backwash system.

For condos with limited space and could not backwash, get 3M AP902