Don't worry about how many stages there are, this is an old ebay trick to make a low end drinking water system with a couple extra cheap filters tacked on sound like the best thing since sliced bread. Worry about the quality of each stage and understand the fewer good stages the better the system will perform, everything you add in front of the membrane has an associated headloss meaning the membrane is less efficient.
Spectrapure has a sale on factory refurbished 4 stage systems that are an excellent reef quality system. They include 1 micron near absolute rated sediment filters, a 1 micron 12000 gallon carbon block, a specially treated and batch tested high rejection rate 90 GPD RO membrane and a full size 20 oz vertical refilable DI filled with their own fresh custom blended SilicaBuster DI resin. They also include a hose thread adapter and an inline pressure gauge. The only thing I would add is a handheld TDS meter for an additional $25. Don't let the refurbished name scare you, these are new systems with all new filters, membranes and resin. The housing or bracket may have a scuff or scratch is all and does not affect their performance.
Others use coarse 5 micron sediment filters, often 5 or 10 micron carbon blocks only good for around 6000 to 9000 gallons, dry untreated and untested off the shelf RO membranes which are less efficient so DI does not last as long and prepackaged bulk DI resin which again does not work as well nor last as long. It all adds up to costing you more in replacements over time. Did you know you can see 40 microns with the unaided human eye? 5 or 10 microns is about as effective as a screen door compared to 0.5 or 1 microns. There really is a difference and Spectraure's almost 30 years in business, and the reef hobby specifically really pay off. They reject more membranes and DI resin in a year than most others combined sell. Their quality control is very strict.
You might also look at Spectrapure systems at places like Marine Depot and Premium Aquatics and save on shipping costs versus directly from Spectrapure.
You cannot go wrong with Buckeye either. Take a look at his BFS-161 Premium system which includes things like a handheld TDS meter and a DI bypass valve so you can fluh TDS creep to waste or make RO only water for drinking and cooking.
