Yes,
You can use the Eagle crank, Balancer and flexplate in a stock or Stage block. The Eagle crank is an internally balanced crank the Stock crank is externally balanced. You can't use the stock balancer or flexplate with the Eagle piece.
Your plan makes sense. Buy good parts you can use to step-up your program at later date when you find a good block. You might consider buying a nice set of Oliver or Carillo rods to build a great rotating assembly, but you would need a set of custom pistons too!
Dave