Just make sure it's a 4 stage RO/DI with a Dow filmtec membrane with a flow rating of 75 gpd or less (higher rejection rate, but, if you have a big tank to service, 100 gpd membranes are made by Dow, just a bit more work for your DI resin depending on the output RO water TDS. Make sure all canisters are standard size (allow you to use cartridges from different manufacturers, so you're not reliant on some proprietary refill/replacement), and preferably clear, so you can see what the cartridges look like. The only big add on might be a booster pump to increase water pressure if your source has a low pressure.
Buy quality and don't get caught up in multiple stage units, unless you have unusual needs. As for nice additions like in-line TDS meters, quick release fittings, pressurized storage tanks, that's all up to you, your needs, and your budget.
BTW, get the TDS meter (handheld Hanna is OK) when you order your RO/DI. It's the only way you'll know whether your water is being processed correctly.
Just pick a reliable company, make sure you are getting a quality membrane (it's worth a few extra bucks to know what membrane you're actually buying, as it's the heart of the system, and the reason you're throwing out good money to make pure water for a fish tank), and don't fall for any name brands using good marketing copy to convince you that their systems are really worth their $300 price.
Henry