Depends on the water quality. Most city water do not need a pre filter as the city already filtered the water to a certain degree before sending it to your house. If you have a well, then a sediment filter is a must to protect the plumbing inside the house.

Depends on usage and the quality of the in coming water. Normally, filter will last for about a year. 

A lot of manufacturers do manufacture filtration system and RO system. In RO system case, most systems are 10″ x 2.5″ filter element. There are, however, some series of RO system that advertise change and click system to change the filter element. While those system provides a great way to change the filter, the filter itself is proprietarily to that manufacturer and therefore the replacement elements must be purchased from that manufacturer, reducing your option.