We recommend prefilters (the filters that treat the water before it reached the RO membrane) be replaced every six months.
With some shopping at an appropriate vendor (aghm), you can find high quality systems that can be built to your specifications if needed, for much less than $300.
Russ
One way to increase the pressure in the holding tank is to use a high pressure auto shut off valve. The pressure in the tank will be about 80% of your tap water pressure.
Standard auto shut off valves shut off at about 66% of tap water pressure.
Russ