People used to swear by RO Man but they've recently gone out of business I think.
My nitrate/chloramine removal filter system comes from Pozzani, they are very helpful on the phone about setting things up and send out replacement bits very efficiently. They also do RO filters but of course I've no idea how good they are, have a look:
Main thing is how many liters per day it can process.
The liters per day is basically how many liters it would produce in a day if it was run non stop.
So if it was 200 liters per day it would produce 8.3 liters per hour, which would take a long while to fill up your tank.
Also it depends on your water pressure, they need a certain water pressure to achieve their stated liters per day, so if your water pressure is low you will either have reduced performance, or need to get a pump to help things along.
As Cathie says, you need a TDS meter to ensure that the water after filtration is pure. You need this whether you make your own or buy it. A pen type TDS meter is cheap from ebay and you just dip it in the water to see how pure it is.
The GPD (gallons per day) rating on an RO unit is under ideal conditions, which means that the water is warmer and under more pressure than it's likely to be in real life. You can half that to get a more realistic GPD figure. I have 100GPD unit, this results in about 2.5 hours to fill a 25L jerry can. I also have a 3 gallon tank though that holds about 15L under pressure so I can actually fill the first 25L can in about 25 minutes. One with a pressure gauge is good as this is an excellent diagnostics tool.
If you're making your own you also need to consider the costs of replacement filters and possibly DI resin. Not too expensive but needs to be done every 6 months.