Mine's three with a DI unit and the TDS reads 0 or 1 after it's run for a little while.
More important than multiple stages, I think it's definately worth investing £20 in a TDS meter - that way you can keep a close eye on what your RO unit is actually doing, and when the membrane might need flushing/changing.
Usually they work directly from mains pressure, I suppose it depends mainly on your local water company and the pressure they supply at.
One quick question for anyone whilst on the subject of TDS meters. At what point would you start to worry about the readings from you unit, i.e above 10 ppm, above 50 pmm. The reason for asking is that I have just purchased a TDS meter this weekend and water direct from the tap is 275 ppm and the RO water 6 ppm.
When the membrance needs replacing (had this one 18months) does the reading go up with a bang or is it gradual and at which point do you start shopping around.
If its RO only (no DI) then I'd say your membrane is working just fine. It'll be a quick process when the membrane gets full. One minute it'll be puttin out like 6 TDS, the next 20 and then prolly shoot up into the 50-75ppm range pretty quick with incoming water that dirty. Remember, try flushing the membrane first if you have a flush kit
Running copper pipes to SUPPLY the RO unit is just fine. RO membranes remove copper from water. However if you plumb it with copper after water has passed through the unit you run the risk of re-introducing copper.