Iam doing up a Black Knight also, not as bad a shape as yours though.
I have to keep the board set original because the owner can't afford the upgrades so Iam having the fault find the components on the boards.
Like the idea of a total new set of boards because it becomes more serviceable in future, especially the power supply.
Keep the old boards, if you decide to sell BK either swap the original boards back in or include the original boards in the sale to keep the purists happy.
Looking forward to the Playfield swap, please take heaps of pics for us to ponder.
Cliff