They're all the same size, except they have a different number of stages. So a 2 stage would be smaller than a 4 stage. Make sure you buy a 4 stage or better for SW. I wouldn't get anything slower than 50 gpd because it's painfully slow!! You can find them new for about $175-225. Used you should be able to find for about $100, BUT beware because often on used ones, or ones that haven't been used for a long time, the cartridges need to be replaced, and they are the expensive part! Check out J&L's pricing for units and cartridges.

I have this unit: http://www.jlaquatics.com/phpstore/s...t_ID=ro-afb050 When I bought it, it was $189 last December. Price sure went up a lot. :(

Here's a price list from J&L:

1 micron (1st stage) - $7.45
Carbon block (2nd stage) - $13.95
50 gpd RO membrane [for my unit] (third stage) - $79.95
Mixed resin DI cartridge (fourth stage) - $31.95

Total - $133.30 +taxes

So keep that in mind if you're buying used. ;)

Do you think RO with DI is necessary or would i be fine just getting an RO unit? There are a lot of cheaper ones on eBay that do just RO with 5 stages. Do you think that's enough?

RO cartridges are only so effective:

Filmtec 50 gpd = 98% rejection rate
Filmtec 75 gpd = 98%
Filmtec 100 gpd = 90%
BFS 150 gpd = 98%

So that means if your tap water is 100 tds, then with a 50 gpd unit your product water will still have 2 ppm tds. That's not good enough. The DI resin will take that 2, and turn it into a 0. IMHO, it is imperative to have 0 tds product water which is why you buy a TDS meter as well, and be sure that your unit is working properly.

Find out the rejection rate on the eBay units before you buy. If you're handy, you can buy a 4 stage RO unit on eBay as long as it has three canisters and the RO. Rerun the hoses, and replace the last one with a DI resin cartridge. Just as long as the order the water travels is micron filter, carbon block, RO, DI. Be sure that the flow restrictor rating is the same as your RO cartridge, and that it's placed inline after the RO cartridge. :)