I'm about to order a custom built RO unit based up a spec I drew up from various Planted/Reef forums. Thought I'd run it by the folk here in case I'm missing anything or getting something wrong:
1 x 1 Micron Sediment Filter
1 x Pentek Chlorplus Carbon Filter
1 x 700ml Deionisation stage 10”
2 x Axeon 150gpd membranes (Formerly RO-Ultratec) membrane (American company).
1 x Membrane Flush Kit
1 x HM Digital DM2 Inline TDS meter
1 x Pressure Gauge with Stem Run Tee on inlet to RO membrane housing
1 x Spare set of filters consisting of (1 x 1 Micron Sediment Filter, 1 x Pentek Chlorplus Carbon Filter, 1 x 1 Litre DI resin).
1 x HM Digital 240ppm TDS calibration fluid.
1 x Self Regulating Booster pump and transformer (to get full production rates and less waste water for volume produced).
3 x Spare Ball Valves for locking in membrane housing when not in use.
1 x Auto shutoff kit
1 x High pressure switch
Some comments for you:
if you have chloramines, you'll want two carbon block prefilters. If you don't have chloramines, you can get a 0.5 mic (20,000 gallon) block for a better price than the block you're suggesting.
Can't tell if you are thinking of running the membranes in parallel or series - either way a standard Aquatec 8800 pump won't be sufficient.
HM Digital doesn't make a 240 ppm calibration fluid (We're an HM Master Distributor).
You may be less than happy with a 300 gpd system and a standard hydraulic shut off valve. If it we're me I'd go with a shut off solinoid that wires in with your pump. Much more reliable.
Make sure you plumb in your pump after the prefilters, but before the pressure gauge.
Make sure you get a liquid filled pressure gauge - the pump will destroy a dry gauge in short order.
Couldn't tell from the order in which you listed things - but make sure your DI stage is after the RO membrane(s).
Make sure you use a refillable DI stage. I would probably go with a larger DI stage or more than one 10" x 2.5" given the amount of RO water you'll be pushing through - especially if you have chloramines.