A twin tank is better than a single, but... for your application I would use the single tank. The twin tank is used primarily for applications that have very high hardness, or very high water usage, or that require 24 hour soft water. Most commercial applications use some form of twin, triple, or quad configuration. The water use of a residentail application is so small that the advantages offered by a twin tank installation are simply not worth it. All modern single tank systems use a computer to calculate the most efficient regeneration day, and all come programmed for middle ofthe night regeneration.
The Clack or Fleck 7000SXt are the best valve on the market, I would not recommend any other. My personal preference is the 7000SXT. Either way, you will not be dissapointed.
Regarding your 40 year old plumbing going bad, soft or hard water, galvanized plumbing has a limited life expectancy. Your Copper repipe will serve you for longer than you will be alive, assuming your pH is under control. Test to make sure your pH is very near or slightly above 7.0 and you will do fine.
For size, a 1.5 Cu. Ft. system with the 7000SXT will do fine. Your hardness is low so the system may only regenerate every 10-15 days, but that is not a problem. You will usually see the 1.5 cubic foot listed as a 48000 grain system. You should set your system to 30,000 grains and use only 6 pounds of salt per CU. Ft per regeneration, or 9 pounds.
Let us know what you decide.