Some answers
Play of the shaft. If it's 1/8 inch side to side, then you need a new cutlass bearing. Fore and aft - I don't know. I would want to have a look at the coupling aft of the engine before getting concerned.
Prop size - the original was 2 bladed with a diameter of 12 inches, and a pitch around 9 inches. I wouldn't worry about the prop until you know how fast the engine and prop will drive the boat, and how the engine behaves on the top end.
If I weren't getting any vibration, and engine will drive the boat at 5 knots, then I would focus elsewhere for a while.
The impellor is behind a plate held down by screws at the lower front of the engine. I'd search the archives here, or better yet buy the book on diesel maintenance on this site for a procedure on how to change it - it's an easy job!
Good luck - you have a great cruising boat!