If you have not made up your mind, ask him his opinion on reusing the pistons. If you buy new pistons, they often come with new pins. If the pins are pressed into the rods, have him install them.
If you reuse your piston's ask him for his opinion on the pins. He may be able to tell you by checking play in the pistons without having to take measurements.
I would install new rings even if you reuse the pistons. Not a lot of money.
Ask him for his opinion on your head. A rebuild head may have had cracks and been welded (may not be reliable). If yours is good, I would have him rebuild it. I have rebuild auto heads just by lapping new valves but they do not run at 7K+ RPM.
You will have to buy specialty tools to reassemble the engine such as ring and valve spring compressors, torque wrenches, possibly tools to align timing chain. If he has them you may want to ask him to reassemble the block and heads.
Can not help you on anything else. I have rebuild many car engines but not Sea Doo Engines. I always bought major brand name parts only. If you use aftermarket, suggest you ask other for their opinion as you are doing. Good luck!