You should be able to source the o-rings locally from an industrial supplier. You will want to use peroxide cured EPDM for any of the rubber o-rings. Those are the most durable in this application. Here in this area we can get them off the shelf from RaleighDurham Rubber and Gasket.
Can you post a photo of the plastic seals you are referring to? They may not be seals but may be spacers for the membranes. If they are seals its almost guaranteed to be PTFE. Also post photos of your interconnect pieces you have. There were two versions of those. The old ones have double o-rings but the newer ones are singles. I'm curious to see which one you have.
$600 is the right price range for a 4" membrane. Make sure they are thin film composite and not cellulose acetate.
The part that goes on the end of the chemical suction lines is called a "foot valve". A totally generic part that you should be able to find pretty easily. Just get the one that fits the tubing size you are using and you'll be fine. Same with strainers. I recommend over-sizing them in regards to the body/line size. You'll get a larger strainer basket. You'll need to get the correct mesh size for what you are trying to keep out. But remember the smaller the mesh the easier it will be to clog. Some good info to get you started here -
https://www.heatric.com/home/heatric-services/services-strainers/guidelines-for-strainers/
I would keep as much sand out of the RAW water pump as possible. Having a long inlet pipe will be a problem and puts the pump at risk of cavitation. That can cause as much damage as debris. If this is going to be a long term solution for you I would recommend using a settling tank for the incoming seawater like your neighbor is using. Ideally you should use a semi-trash pump to draw your seawater into the settling tank or even better if you can make your tank a gravity fill (again like your neighbor). Draw your RO feedwater above where the sand and junk settles. You'll save yourself a LOT of hassle fixing stuff later.
