Here is a link to Amazon with a bunch of whole house filters. You can sometimes find them at big box stores, although they may be separate from the point-of-use filters.
It would be a good idea to go through it and find what is appropriate for you. Are you using a pump or gravity to supply the water? I looked at the manual for one and it made no mention of a minimum pressure, but I'd imagine there must be some impedance to flow which might give undesirable effects if the pressure is already on the low side. The one I looked at mentioned 100 PSI as a maximum.
You mention that the filters you saw don't filter "everything". I'd imagine if you need the highest possible quality then it would need to be a rather high end reverse osmosis (RO) filter. Depending on exactly how your rain collection is set up, and the environment it is in, it may or may not be overkill to use an RO filter system for water that isn't being ingested. I'd imagine it would take a bit of research to figure out what is right for you, and that may be different for people trying to do the same thing under different circumstances.
In any event, I have had some interest in doing the same thing for myself in the near future. I'd be interested to hear about any information you find or paths you decide to take. Good luck!