We did a lot of homework, and then went to our little water district in the states for confirmation. A .5 micron filter will make water safe, but will not remove heavy metals. If the water is brackish, it won't taste great. Based on the information on line, and from our water district, we don't worry about testing the water. We have been using the system for 10+ years now, with no problems. Like you, we use the tap water for brushing teeth, doing dishes, but NOT drinking!
We only filter the water to the little spigot next to the kitchen sink. We haven't ever had to change either filter during a 4 month trip, but again, we only use it for drinking for ourselves and the pets, and cooking. As the filters become less efficient, the water pressure reduces, but we have not experienced that. We change the filters before we head south in the fall.
Sources? General on line searches will find the best deals. Sometimes it's eBay, sometimes not.
We use the standard canisters that are common in the US, but not here in Mexico. Camping World, Walmart, Lowes, everyone has them. The first one is the cheapest 5 micron filter we can get, because, as you said, it gets the chunks out. We used to run a standard .5 micron filter as the second one, but a couple of years ago changed to a .5 micron with coconut carbon. They are pricier, but the water comes out tasting great.
A couple of other tips we have come up with: use velcro to strap the canisters in place, and keep the plastic wrench that is used for taking the tops off sitting on the base of the canister so it doesn't get misplaced, and use soft tubing instead of copper.
