Just to clarify, as you already know, the 800K drive in the Plus hybrid is not spec for a stock 512K. If you decide to restore the 512K, you would be advised to swap the Plus ROMs with the 512K making it a stock 512Ke (or 800K drive kit upgraded 512K). It will be more useful. Youll probably have to retrobright the 512K case anyway as it is unlikely the 512K bucket will match the bezel from the Plus hybrid, even though they may appear close. Of course you can always move the 400K drive over as well, and make it a true 512K.
Also you will have not much left after the "restoration" of the 512K, the Plus logic board will not have ROMs, nor a working analogue board. And depending on the extent of the corrosive damage, the analogue board may not be repairable. You will still have to retro0rite the case to make it presentable and the broken front bezel is not worth preserving. A Mac Plus can be had on eBay for relatively little money, so I don't see this as a hard decision ... Unless you intend to use a hard drive, or Zip drive. In which case you should restore the hybrid in order to have access to SCSI.