When it comes to a RO/DI unti I would get at least a four stage or a five stage unit with a RO membrane rated for 100 gallons per day. In a four stage unit you will want a prefilter as the first stage, a carbon block filter as the second stage, the RO membrane as the third stage, and a DI unit as the fourth. A five stage unit will have two carbon blocks. Make sure to use a carbon block filter and not a granulated carbon filter. A good choice for a RO/DI unit at a very reasonable price is - http://www.melevsreef.com/ro_di.html
$149 for a 5 stage unit and $25 for shipping. You can also find good prices for RO/DI units on Ebay. Compare any prices you find on Ebay to the unit in the link above. I recommend that you stay away from SpectraPure because of my own personal experience with them.
As for the copper, you might want to get your water tested after installing the RO/DI unit to be sure that there is no copper. The RO/DI unit when functioning properly will produce 99.9-100% pure water. My house has pvc plumbing so I haven't had to worry about copper pipes before and therefore can not give you a 100% definitive answer on this. I would have the water tested to ease my mind because copper in very small amounts can kill inverts. Make sure to get a tds meter to monitor the purity of the water that is being produced by the RO/DI unit.
For a reserve tank, most people will just use a rubbermaid brute trashcan with a lid and drill a 3/16" hole in the top of the lid. If there is a tractor supply near you then they also carry a wide variety of water storage containers as well. http://www.mytscstore.com/detail.asp...1034&sonID=157
You can make a cheap, fast and easy drip top off system out of basically anything that will hold water - a one gallon water jug, five gallon bucket, etc. http://www.fragoutpost.com/forums/showthread.php?t=8