I have heard great things about Big Berkey and British Berkefeld filters, and they would fit the bill of a countertop filter system. They're gravity filters so there's no need for any great amount of pressure.

The more particulate matter you clean out before you run the water through the filter the longer the filters will last. But with filtration, long term will be determined how much water you run through them, how clean the source is, the filter capacity, whether or not they can be cleaned and so on. Even after all of that, the more replacement elements you have, the better off you will be for long term filtration.

A filtering pair of Black Berkey Filter Elements will filter 6000 gallons of water (3000 gallons per filter). Long term will depend on how many people you need to filter water for, the volume of water per person per period of time you want to be able to filter and so on.

DIY Big Berkey Filter courtesy of AlphaRubicon and Daire. With this link and a little work you can save a hundred dollars or more. These are not as pretty as the Stainless Steel Big Berkeys, or even the Plastic ones, but what you save in cost can be used to purchase more filters.