This is looking really good, but what a big project, have you started on the building / cistern yet?
Sorry to hear of your incident, but much more is learned from a prototype when it is tested to destruction ;)
My thoughts on the pump are as follows;
First, where are the current pumps? (submersibles in the well?) I only ask because when a pump has a large head on the suction side, they can have trouble priming, and the better you have to seal it to make it self prime, the greater your friction losses.
I missed the double acting bit in your first post, this will be better than single acting, but in any case the flow rate will be highest at mid stroke and zero at dead centres. accelerating and then stopping the mass of water in the pump and pipework every stroke will eat into your efficiency.
An air over water pressure vessel, bladder tank, (whatever you want to call it) on the delivery side will help matters by smoothing the flow in the delivery pipe.
Something similar on the suction side, but with vacuum instead of pressure, MIGHT work.
If the head is not too high, a one way valve bypassing the pump MIGHT allow some water to continue flowing as the pump reaches the end of its stroke (similar to the scavenging that occurs during the valve overlap period of an IC engine) Although if this is going to occur, the pumps own valves should allow it providing they are not too heavily sprung (The valves should not really need springs at all, although some spring pressure does help priming)
Have you considered other types of pump?
A rotary vane pump, provides a smooth flow, and I would imagine is fairly efficient.
I would imagine that you have good reason for choosing a piston pump?
As Ghurd mentioned, 3 cylinders will give a smooth output (most pressure washer pumps for example are 3 cylinder single acting)
If you are set on the idea of a double acting pump, then 2 cylinders will do, with the crankpins at 90 degrees, OR if you have room make a 90 degree V twin, then you only need a single throw crankshaft ;)
Still on reciprocating pumps, have you considered a diaphragm, rather than a cylinder and piston?
Have you considered any auto parts? more specifically a fully floating rear axle from a twin rear wheel truck, if you get a whole axle (weld the diff up) and you have an (approx) 4:1, 90 deg bevel gear assembly.
Taper roller bearings can handle bigger loads than ball bearings (the downside is more friction) but I wouldn't have thought that hub would be a problem even if it is on ball bearings? if they don't last too well replace them with rollers, bearings are also made in several different quality grades, as well as "specials" for abnormal applications.
Copper is used for its malleability, ductility, and conductivity, but you knew that anyway ;) I'm not saying it's not strong enough, but a copper tube with thick enough walls for structural applications, would be uneconomical in terms of price and weight.
Soft solder, I can't find my old stuff, hence using some lead free stuff at the mo, it's rubbish, again not really a structural material.
Hard soldering (brazing and silver soldering) "CAN" produce a stronger joint on steel than welding
("CAN" does not mean it will)
Plastics, is a minefield of a subject, that there are many books on (and no doubt loads of info online)
no point me adding my tuppence worth, as there are so many types that you will need to look at the material you plan to use on it's merits, downfalls, and preferably some raw hard data too.
All I will say is please don't use PVC in contact with your drinking water. (check any others too)
A pump body is by its very nature going to be mostly hollow, to machine it from solid would be expensive and wasteful of materials, a casting would be the usual solution.
You could either outscource the casting, or here is how I would go about
"getting the wife to let you play with fire"
Make her something nice, prepare the pattern in secret, a disposable welding gas cylinder makes a passable crucible if a bit small, A decent wood fire will easily melt aluminium without forced air, and cast it while she is out shopping.
Your profile says NC North Carolina ? Marianne, the girl I made the casting for used to live in Charlotte, but she is now in South Carolina, so you never know, one day I may be in your neck of the woods, IF they will even let me in to the US.