I wouldn't use it passed 006 as the filter isn't then working properly and has absorbed as much as is recommended to be considered zerowater. The filters are cheap enough and easy to replace so it really is a false economy to go beyond that, and the only way you will find out at what level above that is safe will be when you get dead shrimps - it's not worth the gamble in my view!
Oh no, don't use that water than. Or convert the barrel by covering it with nylon mesh (fly screen).
If you are not sure how polluted the rain water is around you, collect some in a clean container, and check the TDS. Anything less than 5 TDS is clean and safe to use in aquariums. Collect it out in the open to get an accurate readout, cause as soon as it hits the roof and gutters, TDS goes up.
I agree, I'm not sure the quality as a filtered water, I never even drink out of it, I always drink bottle water.
As for the filtered water at 006, it still can be used, and it still filtered water, so even at 006, it should be ok for a while.
You can even buy from amazon, the prices do vary. I'm waiting for delivery for a 10 cut jug and 3 replacement filter from www.taobao.cn, the prices is slightly cheaper that purchase 3 replacement filter on it's own.
I'm not all that sure that pure RO or Zerowater is all that healthy to drink all the time as when it is filtered it removes everything, good and bad but I could be wrong as I am not known for my healthy eating........... Even for the shrimps you need to add back the good bits with GH+ (or GH/KH+)?
I had a quick look and most of the range is available from walmart and the prices are the same (near enough) to the manufacturers website!
Bear in mind that water going through the filter of this size is a SLOW business so if you are going to multi-use it, or do many litres each time you are best to get a largish one as kms states, so you can just fill the top bit and leave it to do its thing! If you do get a larger one you may also get a TDS meter (hand as a spare). You need to change the filter before it gets to TDS 006 and in my experience that happens suddenly so as soon as you get ANY TDS you will need to keep a closer eye on it (ie I probably get 90L at 000, then 6L at 001, then 4L at 002 and then TDS goes up each litre - hope that makes sense?).