You know looking at the price of the ready to go kits around $4K sure seems like you can piece together your own kit sourcing parts from non-marine places for many of the parts for about half of that. I have not sat down and gone through this exercise yet but I will try.

A add on system is simply that...a band aid at best...

I'll add this to nose cones amplify anytime walk or handling problems

So I will take the full Hydro plung. What size cylinders (Piston Diameter and Stroke Length) do you all recommend?

You may want to check with CP Performance for a complete system

For a single engine boat, get the longest stroke rams you can fit. It will make docking easier.
Most steering companies will have diagrams on their websites with the needed measurements.

Go drive a cheap craftsman lawn tractor around your yard and get a feel of the steering wheel and the play in it. Then get in a new corvette and do the same. That is the difference between the two. Jeff wurl