Assuming the 64GB board is functional minus the missing chips, that is the board I would focus on to repair. Take the parts you need from the 16GB board and move them over. The TouchID is paired to the CPU so whatever board you focus on you have to insure that those two are together. I would also move the NAND and Baseband IC. So unless you have a functioning CPU/NAND/Baseband/TouchID, then this project will not work out for you.
You are asking for a lot of explicit details on a very difficult repair. I would say if you already have the skills and tools to move micro-bga chips, then surely you have the knowledge to know where to find schematics and board view files for the IP5S and you don't need us to do the work for you.
If you have never done micro-soldering/BGA work, then I would advise you to stop now before you destroy the logic boards even further.
EDIT (just saw Tom's reply but I had already typed this and didn't want to delete it)
If @tomchai tells you it's a waste of time, then truly, it's a waste of time.