Regarding your 2nd point, most of the cartridges use a "Welded Housing" and carry the "Dow Filmtec" brand. However there are sellers on ebay that supply housing that can be opened and you can fill your own filter material. I'm a little skeptical on taking this route without an understanding or testing ability of what effect it would have on drinking water. I wonder if there's an off-the-shelf complete water test kit or a lab that would do that for me.
I live in Taiwan and currently get drinking water from fill-up stations outside due to heavy minerals in the groundwater here. It's not too inconvenient to do it once a week or so, and the water stations are ubiquitous, but I miss having drinking water from the tap.
I've visited water filter shops 2-3 times with the intent of buying a filtration system but am put off by the cost of replacement filters which I know I will have to buy frequently. I'm being cheap but the engineer in me keeps saying there's a better way.
Also, I was not able to figure out how I can test the water quality without sending it to a lab in the US. Do you test your own water?
The additional filter would look something like this [1]. It's an inline sediment filter with a separate casing. For the rest of the unit, you might still be able to buy a complete kit off ebay e.g. [2] & assemble it yourself [3].
[1] http://www.osmiowater.co.uk/media/catalog/product/cache/1/im...
[2] http://www.ebay.com/itm/RO-Zoi-Xi-Pure-Filter-Kit-incl-UV-Bu...