I have the Kent RO system. Mine is a 35gpd, and I use it to maintain a 180g reef, a 30g FOWLR, a 24g reef, and a 55g African Cichlid tank, so I'd say for a 29g, anything will be fine.
The thing about RO units is that even though there is a plethora of companies out there that hock the units, the cartridges and membranes are made by a handful of companies, so they are all pretty standard. You can get just about any RODI system and anyones replacement cartridges will fit them.
We can't discuss competing sites here. If you go to google shopping and type RODI you should be able to find something for a decent price.