Can I use just DI?
A DI stage by itself (without the other RO filter stages) will produce water that is pretty much free of dissolved solids. However, DI resin is fairly expensive and will last only about 1/20th as long when used by itself. If you're only going to buy RO or DI, go for the RO unless only small amounts of purified water are needed.
Do I need a ½ micron sediment filter?
Opinions vary on whether filtering the input water finer than 5 microns has any value. Finer filtration presumably helps prevent clogging of the RO membrane, but there is a cost in terms of dollars and pressure loss. The pressure loss is particularly a problem if you have low water pressure.
Each sediment and carbon stage should be finer than the one before it. For example, a 5-micron sediment filter in front of a 1-micron carbon block will work fine, but using a 1-micron sediment filter in front of a 5-micron carbon block is not advisable.
Where's the value in a 7-stage filter?
Duplicating stages can extend their life or improve their efficiency. For example, if there are two DI stages in series, one can be replaced when it's exhausted without producing any impure water. If both a 5-micron sediment filter and a 1-micron filter are used, they will take longer to clog up. If there are two carbon stages, there will be less chlorine attacking the TFC membrane. Whether the extra stages are worth the extra money is largely a matter of circumstance and opinion (they're more useful if you use a lot of water).
Do I care about GPD?
RO/DI capacities are measured in gallons per day (GPD), typically in the 25 -100 GPD range. The main difference between them is the size or permeability of the RO membrane. Other differences are:
(a) The flow restrictor that determines how much waste water is produced, which must match the membrane, and
(b) The water gets less contact time in the carbon and DI stages in high-GPD units than low-GPD units.
As the GPD rating increases, the purity of the water produced by the RO membrane declines. Membranes above 35 GPD are typically constructed by welding two smaller membranes, meaning there's a seam. 100 GPD membranes are typically more permeable, with a lower rejection rate. The DI stage will make up the difference by removing the remaining impurities but that affects the life of the DI resin.
Most aquarists won't use more than 25 GPD averaged over time. If a decent size storage container is used, that size should be adequate. A higher GPD rating comes in handy, however, when filling a large tank for the first time or in emergencies when a lot of water is necessary in a hurry.
The advertised GPD values assume ideal conditions, notably optimum water pressure (65 PSI) and temperature (70°F). The purity of your tap water also affects it. In other words, your mileage will vary.
What if I have chloramine in my water?
Some water agencies add chloramine (a mix of ammonia and chlorine) to disinfect drinking water. That's fine, except some carbon blocks are inadequate to neutralize chloramine, so it damages your TFC membrane. It can also pass right through an RO membrane and DI resin, yielding ammonia in the resultant "pure" water. This is particularly a problem with high-GPD units.
To find out if you have chloramine in the water, check with your local water company. Chloramine use is particularly common in large municipalities.
If chloramine is present in your water supply, this should be discussed with the vendor prior to purchasing a system. The vendor may recommend a second carbon stage, a "catalytic" type of carbon filter, or a lower-GPD unit. At the time of this writing, the single best solution is not yet clear and a combination may be required. In any case, don't trust a vendor who isn't familiar with the problem.