Most common systems all have the 10 inch housings for the prefilters, usually two or three, then the RO membrane and finally the DI cartridge which in the case of the system you're looking at is mounted horizontally along side the RO filter. Not sure why, but I recall reading that it is better to have the DI cartridge mounted vertically like the prefilters. Also nice to have all clear housings although that adds to the cost and the clear housings are a bit more fragile.
Here's the system I opted for from BWI in Ontario:
http://bwiplumbing.com/ca/index.php/...ming-soon.html
Although it says out of stock and coming soon, it didn't take too long to get mine and it seems I ended up with a five stage vice the 4 stage I paid for. The three cartridges below the RO membrane contain an extra carbon element and I think one is supposed to hold the DI in the 4 stage system. In any case, my DI is in a separate clear vertical housing on it's own bracket. I just mounted both to a 2x4 so it's fairly easy to carry around. Not sure I want to mount it permanently anywhere just yet.
The price is more than the initial listing on the eBay unit you're looking at and it doesn't have the storage tank, but by the time you do the exchange rate, shipping charges and possible brokerage fees, that number will climb in a hurry and the CAN $ price of the BWI unit all of a sudden doesn't look so bad in comparison. Granted, you'll have to pay GST and shipping with BWI as well, but I don't think shipping is that much if you're not in a hurry. Finally, not sure 6 stages is required for your application, might be overkill, but up to you I guess.