I installed a R/O last Oct. for our kitchen use. It makes about 3 quarts per minute, uses only a little for flushing. It is supposed to have a membrane with small enough pores to block bacteria [but I would NOT expect it to do the same for a virus, which, I suppose, you could get in the water from bird droppings on that roof]. However, this unit was right at $3000 so is a lot more expensive than the one you've looked at.
The R/O systems depend on charcoal filters for removal of chlorine and like chemicals.
All filters, R/O or any other, need to have the particulate material removed prior to the filter unit. For 20 yr., we used rainwater off a steel roof and stored in a 2200 homemade cistern [concrete block on a concrete pad and lined inside with a cistern liner, which can be ordered custom made from many sources...for our big tank, a new one last yr. ran us $350 I think it was]. You WILL get a lot of dust, dirt, dead plant material, insects, etc. wash off the roof into a cistern or barrel or any other storage. The biggest challenge I had was figuring out a good and efficient way to filter those things out. I ran gutter downspout and elbows into the cistern, from the roof, so the 1st line of defense was simple screen material at the downspouts to block the larger stuff. I had to clean out the gutter and screens every month or so.
Then the water ran through some filter medium I set in a concrete box I built on the side of the cistern, and the water then ran into the cistern from there. Then, our intake line to the house supply, 3/4 inch poly, had a 1 micron screen on it. Then, just before the pressure pump, we have a simple, inline filter to make sure we got everything we could, as any tiny thing in the water can damage those pressure pumps.
Then, we had a carbon-filter unit at the sink with a separate faucet unit for drinking and cooking water from that....like most of those units come with these days.
We had to go to a well last yr. [legal reasons in our state], and just hooked the well line into the cistern and bypassed the old filter stuff as the well water is extremely clean. But, it's hard, so the reverse osmosis system.
I would have stayed with the rainwater/cistern system if at all possible. It does take some labor, regularly, to keep it all clean. Add a little bleach now and then. Sometimes, any system can be overwhelmed in, say, a thunderstorm, so be smarter than me and design yourself a good automatic bypass for when too much rain is falling. I had some real hassles with that.
