Ok, I have a good idea of what you're wanting to do here. First off, scrap the UV sterilizer. They are more useful in setups where multiple tanks are all plumbed through a single central pump system like in pet shops. It really won't be of that much use to you and it will end up zapping a lot of the desired nutrients that you want in the tank for your plants.
The cannister filters are good to have around for when they are needed. In a tank as large as yours you will not need to run mechanical filtration all the time if you allow it to cycle properly in the beginning. If you run mechanical filters 24/7 they would just end up becoming nitrate factories. I would go with a closed loop system to create a nice current in the tank and only run the filters when you need to.
The auto top-off system is an ok idea - they
do sometimes malfunction and flood the tank until you catch it and shut off the water so keep this in mind when you are deciding where to place the tank. It is not as bad of a problem for fresh water tanks as it is for saltwater tanks. I would forget the idea of mixing RO and tap - that just defeats the purpose of filtering the water in the first place - and decide to go with one or the other.
Jay suggests tap water as the better source, but before deciding on that I would read this post:
http://www.fishtankforums.com/freshw...ttled-tap.html I made on tap water and check in with your local water company as to their filtration practices to see whats in your tap water. When dealing with RO units you need to keep in mind that they produce a LOT of waste water and you will need to find some way of getting rid of it. (Takes around 3-4 gallons of tap water to produce 1 gallon of RO water.) not to mention that you will need to change out the filters after every so many gallons.
If you plan on using a top-off system then you might want to consider using a sump in your design in order to keep a lot of unsightly equipment from hanging in the tank.
I can offer suggestions, advice, and help with planning the plumbing on most of your choices as I have studied it all a great deal in preparing for my next big tank. Once you decide on a game plan, lets get together and discuss specifics.
If you have any questions on anything I have said in this post, please ask and I will be happy to explain in more detail. Let me know if I missed anything here.