Great post Road.
Just food for thought on the typical filter you'd find at camping shops: We bought a Katadyn base camp for our Alaska trip and had to spend hundreds of dollars on filters int he first 10 days. It's supposed to have a very high flow rate to filter water for larger groups (or in an overlanding application, filter lots of water when you are near it so you can be away from it for a few days). If you are filtering tap water for some reason, the Katadyn might work, but for any streams or wild sources the filters just fail or clog up way too fast. We tried pre-filtering and it helped, but we eventually got so frustrated that we gave up on using it and instead resorted to boiling everything after running it through a clean sock. Not ideal.
We are strongly in favour of "Source Filtering" for our permanent set up -- by that we mean the the water is filtered last, at the source of use for the water (just before the tap). This lesson was learned the hard way -- the Katadyn filter's poor performance meant that we compromised our entire water storage system in Alaska twice. We had to stop for bleach (Which we didn't have with us) and de-contaminate our water containers. On one occasion, the Katadyn let algae through which bloomed overnight into a foul, disgusting sewer-like smell. On another, there was clear silt in the container we use to store the water. If silt got through, we are fairly certain microscopic bugs could too! These were two different filters, at $45 USD a pop, and both only lasted about 6 gallons before failing to flow any water at all. With a "Source Filter" system, it doesn't matter too much if your system gets a bit of contamination, as the water from the tap will be freshly filtered. We plan to pre-filter with a Jerry Can , either a rigged up system or a commercial one similar to what Road posted.
I will never buy another Katadyn product based on our experience in Alaska.
For our rig, we plan to spend a bit on our system as we want it built in and easy to use. We are leaning towards a system like this one; it's self contained and it proven for Grecy's trip:
DIY 4x4 Drinking Water Tank, Pump, Filter and Treat | The Road Chose Me
Currently, we are using the katadyn bag from our failed Alaska filter, and have a 5 feet of food-safe tubing into a Sawyer Mini, and then into the Jerry Can or Dromedary. This is the same concept as the Katadyn Basecamp, but the Sawyer Mini filters are far more reliable Fill the bag with water, clip it to the roof rack, and gravity forces water through the filter at a reasonable speed. Most of the time in the last year or so we have been travelling through places where potable water is the norm, so when we stop for gas we often ask "Can we refill our water can?" and folks have always been very accommodating.
I have used MSR filters and they are handy for backpacking and the filters were really good, but their backpacker style one that I had (it screwed directly onto a Nalgene; it was stolen so I don't recall the model) took 5 minutes per liter in real world use. A normal jerry can is about 22 litres so they just don't have the output to be efficient.