As far as heating the water, use a small diameter copper pipe, fed from your Supply, with a petcock valve. Adjust the petcock to just flow enough water to get it near boiling with a small fire heating the copper pipe arranged in a coil...Just enough flow to allow the 200+ degree water output into a pot on the fire. Very efficient, and you can see the water boiling in the pot for the requisite time.

Arrange the "suction" for the Copper pipe to be off the lowest part of the "Settling Drum" to keep particulates to a minimum. Simple and low tech.

I am using a reinforced 275 gallon TOTE from work. These are quite sizable to collect a good bit of rain for the dry times of the year.

It is important to paint the container such that algee does not grow in there. Plus a cup full of bleach thrown in there is what is needed if you plan on drinking some of the stored rain water.

I use a two plastic drum system, one for collecting and one for use.
One drum is in a plywood cover(box) and the other is painted. The paint for plastic lawn furniture works reasonably well.
Does help with the green slime.

I collect in one, then transfer with a pool pump w/filter to the other use drum.
Bleach is added in both.
Both drums have bottom drain for cleaning.

My shed roof is asphalt shingles, and it seems that collected debris at the down spout screen tends to do a "earth pre-filter", as the water is very clean after going thru a couple of inches of asphalt grains, dirt, leaves, sticks etc.
The down spout has a 'first flush", so more debris gets flushed down the pipe. Has dribble drain for after the rainfall.

As the biggest use has been a shower and washing up, I only treat (boil) any water for cooking and coffee, most any other water is in a beer can....if you know what I mean.

As we now have a well, and septic, I just use it for washing up at the shed, but all components are in place and still usable as a shower.
I did notice that I may have to replace the down spout, as it seems the PVC has a crack and small piece missing.
Of course it has been there for at least 10+ years.

Found this regarding the safety of rainwater collected from roofs. (I'm sure that with proper filtering and purification these concerns can be taken care of):

Roof-collected rainwater fails health test
http://www.physorg.com/news88268999.html

Drinking water from your roof carries a significant risk of illness, according to a five-year study from Massey University.

More than half of 560 samples from private dwellings in New Zealand exceeded the minimal standards for contamination and 30 percent showed evidence of heavy faecal contamination.

"I'm utterly amazed at the number of roof water supplies that fail the New Zealand drinking water standards," says Stan Abbott, a microbiologist at the Institute of Food, Nutrition and Human Health.

Roof-collected rainwater consumption is popular because the public believes that rainwater is pure and safe to drink, says Mr Abbott, who is Director of the Roof Water Research Centre at Massey's Wellington campus.

More than 400,000 New Zealanders depend on roof-collected rainwater systems for their drinking water, especially those living on farms, lifestyle blocks or baches that are not served by town water supplies.

The likely sources of the contamination were faecal material deposited by birds, frogs, rodents and possums, and dead animals and insects, either on the roof, gutters, or water tank. Contamination can lead to gastrointestinal diseases from pathogens including salmonella, campylobacter, giardia and cryptosporidium. --- ---

I was born in the really dry part of Oklahoma. We had nothing but cistern (rain) water to drink (the well water was for the livestock and all but undrinkable). I don't remember that my folks (and grandparents) did much of anything to clean or sterilize the cistern water. I also don't recall anyone getting the 'tummies' from it. I do remember my mom hauling some frogs and snakes out of the cistern....d@mn I miss Oklahoma.... NOT!!

Yes people please always filter any water, especially if it come from your roof with bird $&*# on it and such but filter and or boil or do something to it before you drink it.

We will not drink our collected water unless it is treated first.

When we moved to our place a couple of years ago one of my first purchases was my Burkey Water filter and I love it!. It filters out just about everything and the water tastes great. A couple of years ago we had a hailstorm that totaled out our roof. I managed to talk the husband into putting on a metal roof so now that is in place and is a great surface for collecting rain water. Next project is to set up rain water catchment for my garden use.
Doc nothing is worse than walking outside to check on the animals at night and stepping on a frog! Part of the joy of living in Oklahoma.