You need a 40" champ housing for the RO. It doesnt matter if you get 10 or 20" prefilters, the 20" just last longer, Gardiners for Fibredyne pre filter that is a carbon and sediment in one, so you only need one filter housing.
If you have a solenoid on your thank then the clarke CBM240E would be an ideal booster pump, it will automatically shut off when the solenoid shuts