The prices you mention for RO and water softener are pretty high. The RO is particularly high - you should be able to get a similar system for under $500 installed. I have mine serviced every six months for around $100, so the service you're quoted isn't too bad. You could DIY change the filters if you don't mind getting your hands dirty. The softener is in the range of a high-end, high-flow system. I think mine cost around $1500 seven years ago, and at the time had a top of the line valve. That system supports 4.5 bathrooms, so if your requirements are less you could spend a lot less on a smaller system. In general with softeners most companies assemble their own from off the shelf parts, so you want to look at the brand name of the valve, not the retailer. Anyway, I'd recommend getting a few more quotes. Especially from local places that don't sell equipment with their own brand name on it, which will lead to a sizable markup.

Your missing part is a FLOW RESTRICTOR and should be replaced when ever the membrane is replaced. It regulates the water flow to drain, not air. If the old one is still in there you can remove the old one and clean it carefully... it clogs with debris. The shut-off valve pictured above is exactly that... a shut-off valve. It installs in the tubing that supplies water to the RO so you can sht-off the water going to the RO in case you need to service the RO... as you do now. If the tubing that supplies water to the RO is 1/4" PE then you need a 1/4" shut-off valve. If the tubing that supplies water to the RO is 3/8" PE tubing then you need a 3/8" shut-off valve. Home Depot carries them. Also. mind what I said about a saddle valve. If the line supplying water to the RO is hooked up with a saddle valve REPLACE it with a conventional connection. Saddle valves will leak sooner or later and are a giant water leak waiting to happen. If any of this confuses you, then get help. This is important.

@seosmp did you ever get your beverage faucet for your RO system? We are considering the same formour kitchen remodel. If you did would you mind posting a picture of what you choose.

There are several ways that an RO system can be configured, so some of the generalizations above are not truly general. One can simply feed water to an RO membrane (after prefiltering out junk and chlorine with a carbon filter) and send the RO output to a faucet and the waste (brine) to a drain. This is the minimal configuration. The next level of complexity would add a pressure tank to the RO side. This will store RO water, improving flow when needed, but the RO efficiency will drop as the back pressure on the membrane increases. An elaboration that I use, primarily with parts I've purchased from SpectraPure, makes use of a peristaltic pump driven by the waste water. This pump re-establishes the RO water pressure to the tank while effectively lowering it to near zero on the membrane side for efficient filter operation. The pump doesn't use electricity directly, but low-voltage is used by the pressure sensor control of the valve that allows water to the membrane. My house water at this writing is 96 psi and my RO water is at 64 psi. The RO water is plumbed to various places in the house. The RO membranes don't really like 96 psi, so a mechanical pressure limiter is used to set the input at the filters to around 60 psi. Even more elaborate systems can be engineered recirculating the brine and thereby maximizing extraction efficiency. kas Here is a link that might be useful: Spectrapure