A RODI unit and its component are based off your water supply. Are you on a well or city water etc.. Do they treat the city water with certain chemicals etc.. below is a good breakdown of the BRS RODI units.
4 Stage Value RO/DI Systems - Perfect for city water that is treated with low amounts of chlorine, or well water with very minimal contaminants. Four stage RODI systems are a great solution if you only connect your RODI system temporarily or have little space to dedicate to storing a RODI system.
5 Stage Premium RO/DI Systems - Our most popular systems handle common impurities found in well water along with most types of city water treated with chlorine including small amounts of chloramines.
6 Stage Deluxe RO/DI Systems - If you are not sure what is in your water, or it contains excessive amounts of TDS or hard to remove contaminants like chloramines, phosphates, nitrates, silicates, etc. The BRS 6 Stage systems are designed to effectively treat any water your home may have prior to using it in your aquarium. The Dual DI stage helps with the hard to remove ions like ammonia, phosphates, silicates, etc.
7 Stage Pro RO/DI Systems - Perfect for almost any water condition, but if you have contaminants that use excessive amounts of DI resin, the 7 Stage RODI system is the ideal choice. It uses cation and anion single bed resins that give you added control and cost savings over traditionally used mixed bed resins. Most water supplies tend to exhaust either the positively or negatively charged resin more quickly than the other, and by separating the standard mixed bed resin into single beds, you are no longer throwing away good deionization resin.
Here is the link to the 4 stage unit which may be a great place for you to start.
https://www.bulkreefsupply.com/4-stage-value-plus-ro-di-system-bulk-reef-supply.html
Also in terms of a mixing station, a 20g Brute trash can is a great place to start with a first time mixing station. Also fairly cheap for less than $30 and you can get a wheel dolly for it too.